Blizzard posted this news today, through all their usual channels.
Head over to the Fan Art, Original Song, and Movie contest pages to see the submission requirements, read the official rules, and check out the prizes. Information and online sign-ups for the BlizzCon 2010 Dance and Costume contests will be coming soon, so keep an eye on http://www.blizzcon.com for details.
Fan Art Contest: http://us.blizzard.com/en-us/community/contests/blizzcon10-fanart/index.html
Original Song Contest: http://us.blizzard.com/en-us/community/contests/blizzcon10-song/index.html
Movie Contest: http://us.blizzard.com/en-us/community/contests/blizzcon10-movie/index.html
Also note that the deadline to enter this year’s Blizzard FanFic contest is drawing near: August 23rd is the last day to enter.