I'm not expecting Diablo 3 before next year sometime. The goal for them is to have Cataclysm implemented in WoW by the end of this year. Starcraft 2 can very well come before that. But who the hell knows ^_^
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There are retailers taking preorders for Starcraft 2 for May 1st, it could be pushed back but doubt that they would take preorders for it if its not coming out this year.