A tipster's put us on to this casting call, supposedly available on subscribers-only industry site Breakdown Express (he's a subscriber, you see). It lists a voice-acting job, working for Valve. Working for Valve doing Portal 2 work. Said work begins next month, and in briefing potential actors for the role, a ton of details on what must be one of the game's major characters is revealed. If you don't like having this kind of thing SPOILED, move along. Move along. If you wouldn't mind taking a peek inside the sequel's world, though, click through for the character description, along with some concept art of the guy in question, just in case you're a budding voice actor and would like to get your tone just right.
Snappy portrait! Cave Johnson, now-dead boss of Aperture (who you'll already know of if you ever accessed the Aperture website and had a look around), is playing such a major role that he's described as the "principle antagonist", it suggest that Portal 2 may (may) be a prequel of sorts. Which would make sense from a story-telling point of view, what with the way Portal 1 ends and all. Also interesting that he's described initially as a "sidekick", implying that you may be playing the part of a fellow executive/Aperture employee this time around, rather than a science experiment with a pair of robot chicken legs.
Course, it could also mean Johnson is already dead in this game, with his consciousness living on as a prototypical GlaDOS, but if we start down that imaginative road we'll never find our way back...