I'm surprised no one said corn and corn-based foods. That is actually uniquely "American" (if you correctly identify "American" as referring to both North and South America). Corn did not exist in Europe until explorers brought it back.
As for uniquely USA-based food, its kind of dissappointing to try to make a list, but that's okay. A lot of cultures are not particularly known for their good eats.
Anyways, I'm glad more Americans are opening up to other cuisines, though.
This makes me hungry. I think I'll go to the Pho place for lunch today.