I've noticed a few times when I go back into the Starcraft II, it freezes for a good 10 seconds or so even if I'm in game or in menu. For a few people, its worse.
Not sure why, but it seems to mainly affect ATI cards. It doesn't seem to matter how high your details levels are either.
By default, the SC2 uses the "Fullscreen" mode in the graphics setting. Change this to "Windowed (Fullscreen)" and it'll ALT+Tab like a charm!
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While you're out of the game, Blizzard has confirmed a bug that causes the videocard to overheat. See here to prevent your video card from self destructing.