Theres two things I wanted to summerize in this thread.
First, that if you open up your Beta client, and log in with a non-beta account, you will load all the Cataclysm areas and be playing on the non-cataclysm servers. You will appear floating in the air.
Second, is that this works well in Orgrimmar (The place I tested it). You will appear floating or sinking to all the other players who see you, and some may mention it. The height difference is minimal, but still there.
The other thing I wanted to mention, is that there is now a Help GM section in the console on the PTR. MY GAWSH its underwhelming.(EDIT: It appears to already be removed. It now give "NONE" instead of the one command - bgloadbandwidth). There is also a new command - showplayer. Typing "showplayer" into the console hides your character from your own view, making you and all spells you cast invisible to you.
Lastly, I'm talking about BLIZZARD PTR servers, not any private realm or private server which I'm somehow aware that people will come in here saying "No emulator things, k mang?".
Also, one last bulletin: /afk \n not longer disconnects you. It just makes you afk =(. Seems blizzard got a hold of that.