StarCraft II’s menu screens aren’t framerate-limited, which means that when there’s nothing else happening, your graphics hardware renders the shit out of those screens, causing some bad overheating problems. Not to worry though the fix-up is easy, requiring players to add only a few of the following lines to your “Documents\StarCraft II Beta\variables.txt” “Documents\StarCraft II\variables.txt” file (nice catch, commenter LegolElf):

frameratecapglue=30
frameratecap=60