Blizzard has put a website form live allowing attendees of last year's BlizzCon event to register the beta keys they received.
There's no official confirmation of what game the beta keys are for. However, it's a dead cert to be StarCraft II, since Blizzard mentioned a beta test for its RTS sequel was imminent back in February.
StarCraft II has been split into a trilogy of compatible but standalone releases, in the style of NCsoft's online RPG Guild Wars. The first part of the StarCraft II Trilogy - Terrans: Wings of Liberty - is expected to launch towards the end of this year (though Blizzard hasn't made the date official yet).
More information at the StarCraft II gamepage.