As much as I'd like to believe that's truth, I'd still think they'd create a new graphic for a release of this magnitude. That Diablo skull image has been used in wallpapers for D2. If the date also coincides with when Battle.net reopened like Trojan said, it may be possible that's kind of an old graphic.
Although it's entirely possible that it is indeed a sign of D3 to come, in which case I'll proceed to run around the room shaking everyone by the shoulders going "woohoo!"