Online Welten recently interview Rob Pardo, Blizzard's Executive Vice President of Game Design, and he confirmed a fourth Blizzard project.
Quote:
A few months ago, you offered a job on the official Blizzard webpage, considering a Next-Gen MMO. A community manager added that it's about a game that hasn't been announced yet... so is there any new information on that, or is Diablo 3 the mentioned Next-Gen MMO?
No, obviously not, since it is not MMO. So we have another dev-team, that is...
So you have still one project...?
Yeah, there is still one "unknown" project...

Our sources have revealed the mystery behind the evil, purple penguin.
The story behind the purple penguin is that an artist on the
Wrath of the Lich King team drew it but had nothing to use it for. However, the Blizzard web team loved it so they wanted to do something interesting with it. The teaser was just a way to totally throw us off the trail.
Source:Online Welten - Rob Pardo Interview