Coincidentally, I just read this in the 5/2013 issue of Consumer Reports ("Ask our experts" section on page 5):
Q: I recently purchased a new car, and the dealer offered undercoating, rustproofing, and a TST paint protection service for $500. I declined, but are any of those services actually worthwhile?
A: You chose wisely. Today's vehicles have good corrosion protection. In fact, according to Consumer Reports reliability surveys, rust problems have almost vanished in modern cars. Standard rust-through warranties for most cars run five years or more, and many will cover you for an unlimited number of miles during that period. As to paint sealant, it's little more than an overpriced wax. Just buy a good protectant from any auto-parts store.