Barrier to entry?

[swirly]

I have an old WoW account that I used back in classic, but have never updated with any expansion. Periodically I debate how hard it would be to reactivate it. Would I need to buy each expansion to be able to level through it's territory or does one just need to buy the current expansion (or Cataclysm when it comes out) and thus get grandfathered into the other content? Anybody know how this works?