So far Blizzard Entertainment will not be at E3 2009, (they also skipped E3 2008) however Activision-Blizzard will present this year. Its somewhat fluffy what it means. There is still a chance for we will see some World of Warcraft, Starcraft 2 or Diablo 3 news update at E3.
Here is an official statement from Blizzard representative:
Fans of World of Warcraft now chase a fresh goal. Thanks to a recent tie-up between Fanta and Xbox, gamers can grab special online prizes at no cost.
Diablo III Season 38 launched with Ethereals back, but players are reporting Error 395002, disconnects, and server trouble.
Diablo 3 Season 38 starts March 27. Here’s what to finish before Ethereal Memory begins, from class choice and stash cleanup to Conquests and Ethereal
They're not going to be there. Like they said, they have Blizzcon.