The TSA has the right to make you prove you are eligible to return to the mainland - that you are either a citizen of the US or a resident alien or that you have a tourist visa. Sometimes they ask, sometimes they don't and just a drivers license will do. But they can ask.

technically you need to be able to prove your citizenship as well as your residency/identity, so i will second this. the certified copy of the birth certificate should be on hand just in case, alongside a government issued picture id. children and babies should have their birth certificates as well. because they can always ask.