There are some free legal roms available from the MAME website which could possibly be part of the final installation ? Or an optional add-on pack?

I'd love to see Shinobi working, as well as Ghouls and Ghosts (I imagine both will be tough to play on the N900 keyboard though). Great games like Pipe Dreams and Puzzle Bobble would be perfect for the N900 though. The likes of 1943 would be perfect in portrait mode, and Gauntlet would be old school fun. Double Dragon will be difficult to control with the keyboard!