So what happens to accounts with only vanilla?

Any confirmation if they are limited to a subset of Cataclysm content?

One of the panel discussions talked briefly on this. They said that the original wow will also experience all the changes that come along with Cataclysm. You would need the expansions for the subsequent races and content.

You still have access to all the zones you previously did. Just now they are going to be in Cataclysm mode, which likely means a lot of free content for them, but they aren't going to get to see Uldum, or any of the new zones/dungeons, etc.

Good stuff, thanks for the answers.