I'd have to say, of all the things suggested (other than the 'fast food' crud), 'cajun' is about the only one I'd agree with as being American.
We've got a similar problem here in Australia, there's really not a whole lot of 'local' dishes. I mean, there's the local wildlife - Kangaroo, Crocodile, Emu, etc.. but that doesn't really count. I think what Australia is building a reputation for in the cuisine department is the merging and enhancing of a lot of different styles, blending european and asian as well as adding that local meat/seafood element to it..
But the same goes for any 'new' country.. finding something to truly call your own (that's not just superficial) can be tricky.