Okay, I'll try to give you guys the low-down (sp) on these new games.
Basically...
It has all 151 Pokemon, including the evolutions and pre-evolutions for the Kanto Pokemon that were in Johto -- Cleffa, Igglybuff, Politoed, Umbreon, Espeon, Bellossom, Blissey, Scizor, Steelix, Kingdra, Magby, Smoochum, Slowking, Porygon2, Elekid, Tyrogue, and Pichu. That's a total of 167 Pokemon in Leaf Green/Fire Red/Water Blue. It'll also be updated to correspond with the Ruby/Sapphire and Colosseum games, and it'll feature all of the movies, items, Pokemon, and features that Ruby/Sapphire has, such as Pokemon Contests, and breeding.
Oh yeah, Nintendo and GAME FREAK have also registered "Water Blue".
The rest of the Johto Pokemon will be available in Colosseum. Y'know, Snubbull, Sneasel, Lugia, Celebi, Ledyba, etc. That's a total of 100.
Leaf Green/Fire Red/Water Blue/Ruby/Sapphire/Colosseum = "cross-compatible".
Well, it looks like we know how to get all of the missing Pokemon (all 386), items, berries, features, and the National Pokedex.