[This is about the 2010 Blizzard April Fools. 2011 is covered here.]
As usual, Blizzard has a series of April Fools items around their various sites.
There is the Battle.Net Neural Interface.

A new way to connect!
On the WoW front, they have announced the Equipment Potency EquivalencE Number (E.P.E.E.N.), that includes a graphic measurement of who has more. Not quite up there with last year’s Battle Dance System, but okay.

Now we know whose really is bigger!
The Blizzard site also has some fantastic announcements around two mobile games, Blackthorne 2 and Queen’s Quest.
Over at the Diablo III site there was the X-treme Gamer Blanket.
That is all I have spotted up to this point. There wasn’t anything obvious on the StarCraft site, but they might be too busy with beta and launch plans to play the April Fools game.
Did I miss anything in my morning run through the land of Blizzard? Did they announce that the change of relationship between Blizzard and Activision was just an April Fools? One can only hope.
Addendum
A reader comment from David pointed out that there is also a Deckard Cain GPS Voice Pack page, the old Two-headed Ogre player character story brought to life, and a new Battle.net Matchmaking service to add to the Blizzard April Fools list.

Battle.net's April Fool Page
Such fun. Anything out there exclusive to the EU Blizzard sites?