Hey folks, I am really not sure what this has caused but let me explain:
I've bought a WoD Key yesterday and since I'm not around most of the Day I gave my Acc/Pass to a friend so that she can play the Beta on my account. When she had trouble logging in to battle.net due to a security token I just logged in from work and gave her the Beta Client download link.
Appearently the Beta was installed to her battle.net, she was able to login with her own Acc/Pass and is actually now playing the beta.
I have not tested it if I'm also able to play at the same time but it's pretty neat that it works without using my battle.net account where the Beta is active. She definately don't have the WoD Beta and it is not displayed in her account. The account has no WoD preordered though and it still says MoP upgrade account.
Might be a bug? If yes, you might be able to play the beta with just knowing a friend who can pass you the beta client downloader.
I'll have a look at it as soon as I'm home.
Update: Another friend tested it and couldn't log in but he said he already had a link for the client downloader found via google. (but also had no WoD preordered)
Forget that, somehow... mrs. obvious managed to get on the PTR or something else - took me a hell of pain in the head to sort that out...can't delete this thread but would be nice if a mod could do this.