Diablo III's Rumored 2010 Release Date is Not Very Likely
It has recently surfaced that a Diablo III fan site has jumped to the conclusion that Blizzard has somehow "confirmed" Diablo III's release for 2010.
They've made this assessment based on Blizzard's recent http://www.escapistmagazine.com/news/view/89409-Blizzard-Commits-To-One-Frontline-Release-Per-Year that it is planning to release one "frontline" title per year, just as it has in the past.
Expansion packs like The Burning Crusade and Wrath of the Lich King could easily fall under this very definition, based on both their pricing and the amount of content they have to offer.
While it is more than likely that Starcraft II will be released this year based on the fact that it is going into Beta very soon, as with everything Blizzard has ever done, nothing can be certain until they make the actual announcement.
With that said, even if Starcraft II comes out this year, there's absolutely no guarantee that Diablo III will be their next game.
The fact remains that World of Warcraft remains an extremely popular title, and the possibility for the release of yet another, perhaps final expansion pack is almost certainly guaranteed.
Beyond World of Warcraft, Blizzard has already announced plans for two Starcraft II expansion packs which include Heart of the Swarm (XP1) and Legacy of the Void (XP2) and it's entirely possible that Heart of the Swarm may see release well before Diablo III's release date is ever announced.
I do not believe that Blizzard will release two, much less three games in a single year. To release a World of Warcraft expansion within the same period as Diablo III's release would be to cannibalize their own market.
It's highly unlikely that Blizzard would make such a simple mistake. Even if expansion packs don't count as "frontline" titles, there's absolutely no guarantee that Starcraft II will be released this year.