Thought I would go ahead and make a list of old gen Pokemon in Karos. It's safe to assume their evolutions will appear too. If I missed anything let me know.
Source: http://www.youtube.com/watch?v=WNr6wQfNfK4 Pikachu Golurk Magikarp Munchlax Kirlia Panpour Patrat Litwik Psyduck Audino Weavile (shown VERY briefly against Audino) Mienfoo Dratini
Source: https://www.youtube.com/watch?v=sE0owHB_8qo Mewtwo (Safe assumption) Genesect (May only appear for Mewtwo/Genesect rivalry. May not point at being in main Dex) Ramparados Cloyster Gyrados Machamp Zangoose
Assuming they all have their evolutionary lines, it seems that a minimum of 55 old Pokemon will be appearing. It could be held back a bit with limited evo options (Like no Thunderstones until post game, etc.)
There are three conclusions. Either A: The new Dex has very few new Pokemon B: The new Dex is very big. C: Not all Pokemon in the trailers will be in the main Dex.
On 12/02/12 I said I wouldn't change this until something impossible happened. Now, Earthbound is coming to the US.