Nintendo is being slow with their own games because their primary aim is to sustain interest in the Wii, so drip-feeding VC titles is the way it'll be. Sega and others are just in for the $$$, so they want to get their games out there. It's possible to see what's on the horizon by checking what been rated by the ESRB. Also I suspect it's tricky to hammer out the rights to their old games, and the system-makers cut, which is why 3rd party titles are slow in coming (so far it's mostly Nintendo, Sega, and Hudson first-party stuff)