Minimum content is the way to go on a resignation letter - kranky's letter is the best example. A resignation letter is a legal notice - anything you say in there can (and might) be used against you in a court of law.
In particular, do not give a "reason for leaving" in a resignation letter, especially if it is a negative one. If there are problems at your old employer later, someone may take that information and attempt to accuse you of employment contract violation or sabotage.
For similar reasons, all "exit interview" information should be shared verbally only; do not sign anything for your employer that appears to have information detrimental to your co-workers or supervisor, since you can be sued for such remarks. (Why do you think most companies give employment referrals verbally or with minimum info, if written?)
I know, I know, I sound paranoid, but (as I've noted in previous postings of this sort) I've been a consultant for a long, long time, and I've been screwed over by more than one company in the past because I didn't pay attention to details like that. Learn from my mistakes, as I did.
Lady Niniane