Well, it's always hard to get any sort of meaningful commitment from The Action Team, I mean what do you think we are, The Commitment Team? But I'm signed up for a six month subscription so will be playing through then. My character and Doc are on the same ol' server.

You don't need Cataclysm to see the changes to the vanilla zones. They are just going to flip the switch on 12/7 and the changes will affect everybody. You will need to buy Lich King if you want to play Death Knights or go from level 71-80. You will need to buy Cataclysm if you want to play the new races or go to level 81-85. I could see them putting out some kind of bundle in the future but for now, yes you have to buy them all separately.

If you do decide to check out the changes, start downloading at least a day and a half before you want to play because the patches are MAMMOTH, like 12 gigs. If you don't have the game installed at all then it is much more time-efficient to just download the installer from the Blizzard site and start from scratch than it is to install off your old discs and patch up.