Looks like the N900 might eventually get the ability to play N64 games using the Mupen64 emulator compiled for ARM. Apparently, the big hold up at the moment is a lack of OpenGL ES drivers:
http://www.gp32x.com/board/index.php...ost__p__751625

Unless I'm mistaken, this should work on the N900, and possibly even the N8x0 (I'm going to take a wild guess and say that this would probably be unusably slow).