From the grapevine I have heard that Blizzard has been running the MoP betas on the Cata servers.

I have no solid proof of this, but it has been a recent issue for everyone. It would make sense if blizzard was preloading and testing on the actual servers before release. That and half the community is playing the beta so they had to put it somewhere.

Like I said, Grapevine.

http://www.youtube.com/watch?v=Gs--phzj2TQ

there you go