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.
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.
PTR 4.0.1 Patch Notes