There is the App called NESDroid. (as posted above). I have the HTC Hero (HTC Droid phone from Sprint), and I have seen this app.
There is a catch though. Like any emulator (being sold or not that I've seen) you have to supply your own ROMs, but a quick Google search will bring them up for you.

(Check out Coolroms through Google. That's where I get my games for Project 64 the N64 emulator for PC.)