zidsal
OG
Patch 4.0.1 now on Test Realms
Patch 4.0.1 is now available on test realms (=not beta, the regular test realms). People who aren't in the Cataclysm beta yet can now test most of the Cataclysm features, aside from anything requiring leveling.
US PTR Website
EU PTR Website
This is basically the exact same build as Cataclysm Beta, just without all the Cataclysm-only stuff. Old world changes, new quests, etc ... are NOT AVAILABLE, but will be available on live realms for people without the expansion after it's deployed.
Basically, on the test realms:
Classes changes are up, new talent builds, new glyphs (not fully implemented yet), etc ...
Revamped old zones are NOT available
New graphics (textures, water effects) are NOT available
Archaeology is NOT available
Guild leveling is NOT available
Flying in Azeroth is NOT available
New content is NOT available
I think that's enough to answer 95% of the questions about this.
it would be cool if all the eu twinks went to the same realm (I'm currently at brill) so we can test all the changes to classes, one think to hear about them and another to experience it first hand