Developer ZodTTD (a celebrated modder who has already brought a port of Quake and several other emulation projects to iPhone) has announced on his blog that he's working on an N64 emulator for the iPhone 3GS and iPod touch 3rd Gen - the most powerful iDevices.
The emulator may run on less powerful devices, says ZodTTD, but the inferior innards will slow things down.
Details are slim at the moment. ZodTTD keeps his post short to "balance the hype," and much of what he does divulge is pitched at a technologically literate audience, but he seems confident that it's coming. In his own words, "Things look good for this project".