I am married.  I'm also a lawyer, but (a) I don't practice family law (which includes divorce law), and (b) in any event, no one should rely on this post for legal advice.  Further, no one should rely on this post for conclusive halachic advice -- the halachic issues here are difficult, and anyone needing advice (e.g., contemplating divorce, or contemplating marriage) should _absolutely_ consult a competent and halachically-knowledgeable rabbi.

    Contrary to Jonathan's speculation, I believe it probably is _uncommon_ in U.S. divorces for any agreement to specifically obligate the husband to give the wife a get.  First, it would be difficult to get a U.S. court (state or federal) to grant specific enforcement of the promise (i.e., to issue an order compelling the husband to give the get as he promised), because such an order could be considered an order to perform a religious act, and such an order would violate the Establishment Clause of the First Amendment.  Second, such a specific obligation could create _halachic_ problems -- under the halacha, if a get was given under compulsion of the law, that very fact could render the get invalid.  

    My understanding of how this issue is addressed in the U.S. is that a prenuptial agreement states that, if the couple should (God forbid) get a civil divorce, and if the husband does not give the wife a get within some specific period of time, he would be obligated to pay her some specific amount of support until the time when he does give her a get.  The theory is that U.S. courts would likely treat that as an ordinary financial-terms prenup, and would have no problem ordering him to pay the money as specified, payment of money not normally being considered a religious act.  Further, since the halacha obligates the husband to support his wife (and remember, if there's no get, they're still married under halacha, notwithstanding the civil divorce), the civilly-enforced support clause would not be deemed to be "compulsion" to give a get (as long as the amount specified is not outlandish).

    To respond to Adam, agunot and folks sympathetic to them _have_ sought legal protection in the U.S.  New York State had a "get law" a number of years ago, but it had constitutional issues and halachic issues.  IIRC, the law was amended in order to address those issues.  I have not kept tabs on either statutory or judicial developments in this area; others may wish to supplement what I've said here.

    Because of the importance and delicacy of this issue, I will repeat my caveat and exhortation from above -- anyone needing to take action in this area (e.g., anyone contemplating either divorce or marriage) should _absolutely_ consult a competent and halachically knowledgeable rabbi.