I've had it happen plenty when ALT-TAB'ing, but it's also happened randomly for no reason as well. Yesterday afternoon I was fighting the last set of minibosses in Tam-Tara Deepcroft and I crashed out. With the way the servers currently are I wasn't able to get back in until about 11:30 that same evening.

I figured it was a regression of some sort with a conflict in with 3D drivers. Are you guys also running Nvidia cards? GTX560 here.