Blizzcon 2010 attendees (online and IRL) will receive an exclusive In-Game WoW Pet and SC2 players will receive an exclusive Character Portrait (Murloc Marine) and have special unit model changes made (e.g. decals on your armor.)
If you really want these decals for your SC2 models and you can't attend - don't worry. There will be plenty of other options available when the SC2 store opens and you can have your own choice of $10-25 decals (or some other form of unique model change or modified texture). =]Blizzard might even donate the initial proceeds to charity!
At Blizzcon 2010 (this coming October), there is also supposed to be a "HUGE" Diablo 3 announcement. Game Director Jay Wilson said, "I can say that this Blizzcon, in terms of Diablo news, will be the biggest one that we've ever had."
I really haven't been following Diablo 3 that much, and I don't have any inside contacts that are working on that specific project there so I have no idea what the announcement will be.
But, I am completely expecting to be disappointed.
Here's what I'm thinking off the top of my head (in order of most likely in the unlikely event the news will indeed be "huge"):
1. It better not be anything stupid, like introducing multiple new classes. "Huge news everyone! There are two new classes!".. "Just kidding.. there's THREE! OMG!" *cheers and yelling from the crowd and many erections*
2. Maybe #1 but also throw in some new "features" to promote, like crafting skills, a PVP arena (or other PVP related stuff similar to WoW), new spells (also similar to WoW), hired NPCs, and/or some web and mobile components that are tied into the game.
3. That D3 is ahead of schedule and we'll see it sooner than expected like next year. (They won't give an exact date though.)
4. Diablo 3 will not just release for the PC, but also the PS3 and XBOX360. Yay.
5. Diablo 3 will be an On-Demand gaming platform (improbable though, as it's too soon.)
I can't see there being any other kind of "big news". He also said that it will be the "biggest one we've ever had", so I really hope it's not just the introduction of 1 new class.. but it probably will be.