Duders, I stopped playing WoW before Wrath of the Lich King came out.  I want to buy cataclysm and also play as one of those sweet death knights.  Do you think I'll need to buy both expansions, or will buying Cataclysm be enough?

Without knowing 100%, I bet my cup of coffee on that you need to buy both.

You need them all. Thats how expansions work. 

You'll have to get both.  Edit:  The "cataclysm" will happen to everyone, even people with none of the expansions.  So you'll be able to see the newly redesigned Eastern Kingdoms/Kalimdor, but you'll have to buy WotLK to play as a DK and Cataclysm to be able to do the sutff that requires the Cataclysm expansion (level 85, Cataclysm dungeons, new zones)

both, says on the box

" You need them all. Thats how expansions work.  "

Freaking bloodsuckers at Blizzard.  I figured that's what I would have to do.  At least you still get a free month with the purchase of Wrath...right?

^^nope

You never got a free month with the purchase of Wrath, just Vanilla WoW.  Also... It's extremely unreasonable to expect to get all previous expansions free because a new one came out.  If you don't have the expansion previous to the new one, you're not the right level to do anything in the expansion, so it also wouldn't make sense to only have to buy one expansion and ignore the ones before it.

you need WOTLK expansion to play DK, but even then you'll only be able to play levels 55-60.  Then you need to get The Burning Crusade to level from 60-70.

While it is true that many MMOs give away all previous content with a new expansion (Everquest II and Lord of the Rings Online come to mind) World of Warcraft can pretty much do anything they like. I'm sure when Cataclysm comes out in December they'll release a new Battle Chest with Vanilla, Burning Crusade, and Wrath of the Lich King but that doesn't help anyone who just needs one or two of those. From an infrastructure standpoint I'm sure you need all previous releases simply because of how much has been built into the game. You know you're going to get far far more out of WoW than what you spend on it so put up the money and smile.