For those of you looking to kick it oldskool when the Nokia 770 comes out, there might be a few open source projects that fit the bill. Frodo 4.1b, a Commodore 64 emulator, and Atari800 built and ran without modification on the x86 Maemo development environment. Frodo chewed through almost all of the CPU on my Athlon XP 2500+, so getting that to run on the actual device might be a bit iffy. I saw some .asm files in the Atari800 source, which may be the downfall of Atari800 on Arm.
We’ll see though. I tried a few more modern emulators (gameboy, nes, snes, etc) but nothing looked like it was going to be easy to get work at first glance. Here are a few screenshots of what I was able to get running this evening: