People can have rediculously good hands on their first hand, so don't assume the raiser is full of it. I flopped 4-of-a-kind once in my first hand at a NL table once.

Once at a SnG, I was dealt AK in late position, and everyone (literally) ahead of me called, so I went 1/2 all in. One raised me all-in, and I called. He had 3 4 suited. I doubled up first hand.

Since I know people are less likely to believe a first-hand all-in, I might be tempted to do it with AA/KK. If anyone else goes all-it, I'll give them the benifit of the doubt and fold unless I've got AA. It doesn't cost any money to fold to a bluff pre-flop.