Ok, a bit more speculation.
Last week, Xfire briefly had mention on their site that Blizzard would be announcing a new game at their World Wide Invitational at the end of this month. This was very quickly pulled from the site.
Currently, if you try and visit the following address:
http://www.battle.net/forums/board.aspx ... d3-general you get an error stating that the battle.net forums are currently down. If you go to the main list for forums, all the others are working. Additionally, if you change the forum name to anything else that doesn't exist you are redirected to the main forum list.
There are also splash screens when trying to visit most main Blizzard sites (at the very least Blizzard.com and battle.net have them), with much speculation over what it could be. Considering the only time Blizzard use splash screens on their main sites is in the days leading up to a new game announcement, i do not believe it could be WoW expansion or SC2 related, since both games are already announced.
Lastly, Rob Pardo gave a keynote speech at the Game Developers Conference in Paris a few days ago, he was asked to comment on the rumors that a new game announcement was going to be Diablo 3, in response he said this:
"So you want me to announce the game before our announcement?" he said. "No offense, but I think there's like 300 people here, and I'll be ripped apart by 8000 people there [at the Invitational] if I pre-announce it - but it's going to be really exciting. I think everybody here will be really excited about the announcement."
The Blizzard World Wide Invitational starts on Saturday. Time will tell.
Edit: The 2 main theories is that the splash images either a) depict cracking ice that will eventually reveal Arthas or b) depict a decaying Worldstone that will eventually reveal Diablo.