The GB emulator is based on gnuboy, so in theory every game that works with gnuboy will work on the iRiver.
Once we see a decent RB LCD driver (to exploit all four shades of gray) and once the iRiver's clock speed goes from 11mhz (RB's default) to the full 140mhz, we'll start seeing some actual playability, granted optimizations will probably be needed.
Exciting times ahead!