We kind of already figured this (and who is really jumping into Northrend without first having finished up with Outland anyway?), but Datth has confirmed it on the forums: in order to install Wrath of the Lich King, you'll have to have Burning Crusade installed as well. Not a huge surprise, but just in case you were recruiting someone to come up to level 80 with you, make sure to tell them to buy all three versions of the game when Wrath comes out, because they'll need them.
The bigger question is: when will get a battlechest? It took about seven months for Blizzard to put Burning Crusade and World of Warcraft in the same box, but can we expect them to do it faster with Wrath of the Lich King? Probably not -- while the price may drop a bit on the current Battlechest (if you wanted to jump into the game the day of Wrath's release, it would cost you $70 total, and our guess is that that's about $10 more than Blizzard will end up charging), odds are that anyone getting in now would have to buy that and Wrath to play.
Of course, odds are that you've made up your mind about buying the expansion anyway. Anyone out there still playing with vanilla WoW? As much as Blizzard has been about skipping past all that 1-60 content as fast as possible, I wouldn't be surprised at all if they even tweaked the subscription pricing on that end to try and pull more people into buying the expansions. They already will let you try the game and first expansion for free, and you'd have to think eventually they'll try to convince the last few demographics holding out to join up as well.