I shit you not! well maybe a little bit... its PTR files merged with Beta
I downloaded Cataclysm optimistically a few weeks back, and just for shiggles, i tried logging in, low and behold, I got in, but what I saw was the 4.0 PTR Realmlist
Now none of the new race/class combos and worgen/goblins work (when I tried making them I got a Failure message), so i settled with making a humble undead rogue, Deathknell was the same, but as soon as I got out I saw the Calston estate, the new, gothic architectural Brill and the Plaguewagon filled warzone of Silverpine forest, I asked in /1 if anyone else saw these changes, everyone else said no, there are no geographical changes in the PTR build.
So yea, too see the new world changes download the Cata Beta, login with your usual details for PTR, and volia!
It is buggy, IE it looks like the PTR and Beta are competeing/running together so some of the npcs are flying round or walking in the ground like they're in their old place from Vanilla.