Blizzard has posted the following information on its forums: "Certain screens make your hardware work pretty hard. Screens that are light on detail may make your system overheat if cooling is overall insufficient. This is because the game has nothing to do so it is primarily just working on drawing the screen very quickly. A temporary workaround is to go to your Documents\StarCraft II Beta\variables.txt file and add these lines:
You may replace these numbers if you want to."