> I find it curious that Americans prefer to use the long "going to have
> to" or "gonna have to" versus the much shorter (and equally meaningful,
> methinks) "must". Is there a subtle difference in meaning
> that I am missing here?

It's not just an American / Commonwealth distinction. There's quite a
lot going on here.  A couple of references that might be useful to those
interested in this subject are:

  a pair of papers from CLS (Publications from the Nth Regional
   Meeting, Chicago Linguistic Society, annual (4 < N < 30 (= 1995)))
   volumes that explicate the semantic and syntactic distinctions
   between the (American) English usage of 'will' and 'be going to'
   /g@n@/, written by Robert Binnick. They are called, surprisingly,
   "Will and Be Going To" and "Will and Be Going To II", and they were
   published in the 70's (if memory serves, 6 <= N <= 10).  They should
   be findable in any university library, and can be bought from the
   Society (parenthetically, they're cheap); details available 
   via the link above.

   They're both quite clear and readable, and give plenty of examples.
   And, for *my* money (I'm a linguistics professor [Caution: your money
   may vary; references on request, some assembly required, batteries
   not included]), they are the closest thing to what I could consider
   definitive authority on this topic.

  Chapter 9, "Negation and Modality", in Frawley's _Linguistic
   Semantics_ [1992, Lawrence Erlbaum], which is one of the standard
   textbooks (I just finished teaching with it last week).  It seems to
   me from about a month's attention to a.u.e that a large number of the
   questions and disputes have to do with negation (and negative
   polarity items like 'any( )more') and modality ('must', 'will', 'have
   to', 'going to', subjunctives, et very complex cetera), and
   especially to their interaction.  Negation and modality are very
   closely related to one another in semantics in all kinds of ways.

   Frawley's discussion of them and other topics is very thorough and
   has many, many interesting examples from a lot of languages
   summarizing most of the issues.  Here's the table of contents of
   Frawley:  1. Semantics and Linguistic Semantics:
                Toward Grammatical Meaning
             2. Five Approaches to Meaning
             3. Entities
             4. Events
             5. Thematic Roles [e.g, "Agent", "Patient", "Receiver", etc.]
             6. Space
             7. Aspect
             8. Tense and Time
             9. Modality and Negation
            10. Modification

   I'd reckon it as a good authority for semantics. For English syntax,
   by the way, a very good grammar that's as authoritative as Jesperson
   is McCawley's relatively recent two-volume "The Syntactic Phenomena
   of English" [University of Chicago Press].


  As to this discussion, the usual oppositions are those between

             will - and - be going to  /g@n@/
               |              |
              and            and
               |              |
             must - and - have to      /haeft@/

   a four-way distinction; and as well between the logical classes of

      Deontic   (two-place, social permission/obligation)  modals,
      Epistemic (one-place, logical possibility/necessity) modals;

   and between the logical classes of

      Necessity   (social obligation, universal,   "square") modality,
      Possibility (social permission, existential, "diamond");

   another four-way distinction.  Lacking four-dimensional HyperSpace
   Markup Language, I cannot provide a graphic for the intersection,
   of these two four-way distinctions, alas.

  Will and must are both Necessary-class modal auxiliary verbs, and are,
  like most modals in most languages, ambiguous between Deontic
    a) If he will do it, ... [means "If he's willing to", not simple
                              future will, banned in 'if'-clauses]
    b) Cinderella must be home by midnight.
  and Epistemic:
    a) It will rain tonight.    [certainty in expression of the future;
                                 logical, not social]
    b) This must be the place.  [certainty in expression of
                                 logical conclusion]

  As modal auxiliaries, both will and must have a morphosyntactic
  peculiarity in English: they are defective verbs in that they
  do not possess all the forms ("principal parts") of other verbs.
  There is no infinitive, participle, or gerund; moreover, modal
  auxiliaries may not be inflected for tense.  Since modals cannot
  be inflected, and since all auxiliary verbs in an English verb phrase
  (except the first) must be inflected, whether finite or not, modals
  are limited to first position in the verb phrase (where they must be
  followed by an infinitive).  

  There is also another opposition among the formal auxiliaries, between
  historically "preterite" and historically "present" forms:
                  could                        can
                  might                        may
                  should                       shall
                  would                        will
  Although must is based on a preterite, it has no preterite sense;
  indeed, the "preterite" usages of these forms are quite rare, limited
  to sentences like:
            When I was young, I could do 30 chin-ups;
            now I can only do 29.

  All of which goes to show that one must use other means than
  inflection to express preterity with a modal.  Not really surprising;
  English has been abandoning its old inflections like a snake shedding
  its skin since the Great Vowel Shift, and replacing them with
  periphrasis of one kind or another.

  One variety of such periphrasis is the use of the Perfect have
  [+ Past Participle]:  I should have punched out his lights.
                        I may have discovered it.
  This is becoming an inflection in (American) English, as the frequent
  confusion of "have" with "of" in spelling, and the very idiosyncratic
  phonology of these constructions (like all modal phenomena) attest.

  But it's not the only kind of modal periphrasis, and it doesn't cover
  all the logically and socially necessary cases.  To take up the slack,
  there are a number of idiomatized ("canonical") paraphrases, often
  paradigmatically related to real modal auxiliaries.  

  The canonical paraphrase for will is be going to, idiosyncratically
  (but universally, in America) pronounced /g@n@/, with varying degrees
  of prefixal agreement:     Singular           Plural
                      1     /aNG-g@n@/         /w@R-g@n@/
                      2              /y@R-g@n@/
                      3     /z-g@n@/           /@R-g@n@/

  It has preterite forms, based on those for be, and in this case it
  is more flexible than will, since it can express tense:
        I was going to tell him, but since you already have...
  and thereby provide the means for a nice distinction, as against:
        I would have told him, but since you already have...
  Note, the have solution doesn't work for will itself, only would:
     ?? I will have told him, but since you already have...

  The oddity in juxtaposition of the clauses here is due to the future
  perfect sense of will have, logically Future [Perfective [Vb]] where
  was going to is Perf [Fut [Vb]].  Hence the second clause doesn't make
  sense, since it refers to a different construal of the modality and
  aspect.  would, in its "preterite" sense is equivalent to used to
  /yust@/, another periphrastic construction.  They're all over the

  The canonical paraphrase for must is have to, idiosyncratically (but
  universally, in America) pronounced /haeft@/.  It, too, has inflected
  parts: /haeft@/, /haest@/, /haedt@/, since these are only semantically
  but not (yet) syntactically modal auxiliaries, they appear with
  do-Support when negated:
     You don't have to /haeft@/ do that.  NOT [OBLIGED [Vb]]
     He didn't have to /haeft@/ say that.
  The first of these are very different from the negated modal must:
     You must not do that.                OBLIGED [NOT [Vb]]
  and the second uses both negation and Perfective to provide a very
  different meaning:
     He must not have said that.          NECESSARY [NOT [Perf [Vb]]]

  must must precede a negative or a Perfective have, producing
  NECESSARY/OBLIGED [NOT [V]], and NECESSARY [Perf [Vb]].  The laws of
  causality, and the limits of the imperative mood ("Canute's Law")
  prohibit OBLIGED [Perf [Vb]], and the rules of English syntax prohibit
  most other configurations with true modal auxiliaries. have to
  furnishes a quasi-pseudo-semi-hemi-demi-must for such purposes, and
  an alternative must available for occasional contrasts.

  In sum, it is very useful, one may even say vital, to English speakers
  to have this degree of flexibility in the modality department.
  English may have only remnants of its mood (=modality) inflections
  left, like the various subjunctives, but it has lots more nascent
  morphology that's already almost paradigmatic.  As with the Northern
  Cities Chain Shift, one can see an actual language change in progress

  The keyword for this phenomenon in the linguistic literature, by the
  way, is "grammaticalization".

 -John Lawler                     More grammar
  Linguistics Program   University of Michigan
 "..and, who knows? Maybe the horse will sing."