I have been married for 16 years, and I never believed in pre-nups. But as I get older, and wiser I realize that in some cases a prenup is not only a good idea, it is a requirement. This is especially true in 2nd marriages when kids from a previous marriage are involved. First and foremost you need to ensure that should a marriage end in divorce that your kids do not see their inheritance going to their step parent.
Another cases that was mentioned is a trust fund. If you are entitled to money from a trust fund, you need to have a prenup that specifies that no alimony can come from the trust account.
If you are young and have nothing...no prenup is needed.