What happens is when people with ATI cards(I have 5850) alt+tab, and then try to reload the game you are stuck for 30-45 seconds staring at a black screen where people see you as dropping.
This used to be an issue before because I kept misclicking the windows key instead of alt. I did a work around by disabling the windows key so I would never alt tab by the key however I sometimes forget to turn off msn messenger when I am appear offline and when I do get a message it forcefully alt+Tabs me and it takes forever to reload the game. I have a really good PC so I doubt it is that and I alt+tab of other games just fine. I've noticed with Cata though that the alt+tab times are a bit longer but nothing near the times of sc2. Any resolution?