So you're on the PTR wanting to enjoy all the gooey goodness that is Patch 3.1. You have this urge to go to Dalaran. I mean, it's so pretty right? And there's a bank there, and your hearth stone is probably set to their as well if you don't have a Ring of the Kirin Tor.
There are hundreds of reports coming in from around the net going up on the official Blizzard forums, tons coming into the WoW Insider Tip Line, and the problem has been experienced by every staff member here that's been on the PTR.
You can move no more than a few dozen paces in Dalaran without there being a critical error that causes WoW to crash.
There were some early reported rumors that the problem is linked exclusively to ATI video cards, but I can say beyond a doubt that such is not the case – I have an nVidia GeForce GTX 260 I'm getting crashed every few minutes while in Dalaran.