A few have been confirmed. But we won't know all of the previous ones until the game comes out. Pokmeon like pikachu and leafeon have already been shown.
Update: Yes, you can catch all the previous game's pokemon once you have the National Dex.
Of course I expect the regular geodude, onix, pikachu, machop, abra, ponyta, and psyduck should all be avalable at some point during the game.
