The Activision part may be returning to the bigger, better, more badass E3 this year, but the Blizzard component is not. A rep for the company confirmed to us that Blizzard itself won't be appearing at the show, which I shall now prove with the following quote.
Blizzard Entertainment will not be attending or participating in E3 2009.
See? Those are certain terms, friends. Of course, the annual BlizzCon has practically become a cottage industry in itself, so ol' Blizz already has a ready outlet from which to dispense major announcements on Starcraft, Diablo, and WOW. But BlizzCon is typically later in the year, so if you're scrabbling for news on those games early in the summer, well, you may be out of luck.