I already have one of those devices; it's called a notebook
Seriously though, is there a MAME port underway for the GBA? I can't imagine that working very well, as I could barely even get Pac-Man or Asteroids working at speed on my 486-25 back in the day and the GBA is some kind of 16MHz RISC chip. For that matter, it's pretty crappy on my Zaurus as well and that's got 206MHz in it (I think it's a timing issue though rather than a too-slow processor, because even playing with frameskip 8 seems a little slow.)
I actually like the NES ports of a lot of arcade games, and the NES emulators on the GBA are top-notch. (Granted the NES had better hardware than a lot of 1980-81 era arcade games, so maybe the MAME thing isn't so farfetched after all....)
Rob