Thinking about it further, I'm guessing the existing "basic WoW" retail box will just cease to exist. They'll do one of two things. The first possibility is they'll sell Cataclysm in two flavors: a level-60 capped account version that functions as the new basic WoW installation box and will be priced accordingly, and a level-85 capped account version that functions as the third expansion and will be priced as all the new expansions have been.
The second possibility is they'll sell Cataclysm in just one box. Brand-new players can buy that box and install the whole damn shooting match. When they create their new account, matched with the activation key for Cataclysm, Battle.net will give them a 60-capped account with their 30 day trial. When existing users with TBC and WotLK buy the same box, enter a Cataclysm code, and choose the upgrade account option on Battle.net they'll get the 85-capped account, Worgen and Goblin creation privileges, and so forth. Any player who has the Cataclysm landmasses already installed can upgrade to an 85-capped account by having the other expansions and simply buying the code on Battle.net.
Honestly, the second option sounds much too complicated for your typical marketing effort. They'll probably stick with the first option. This doesn't alleviate the problem of new users somehow starting from Ye Olde 2004 WoW Discs who will still have to download and install some ginormous 5GB Catalyst patch but it'll provide an easy path for the majority of new users who'll be picking up the game fresh from retail outlets.