I'd say the Wii has an 80% chance of hitting 100M. It's almost guaranteed to sell extremely well, and the biggest "if"s are more about the direction first and third party support will take. This can be seen from the shifting arguments of most nay-sayers ("sure it'll sell, but only get spin-offs and casual games!!!1").
Then again we seem to be on different planets about a lot of things. I wouldn't say the PS3 is guaranteed to beat Gamecube/Xbox sales yet (looks likely though), and 50M is the absolute maximum it can ever hope to sell. Outselling the 360 is a 1/3 chance at best.