Actor J.K. Simmons has been confirmed as the voice as Aperture Science president and founder Cave Johnson in Portal 2.
The Law & Order, Red Alert 3, and Spider-man actor will be delivering lines such as the following from the game:
"Those of you who volunteered to be injected with praying mantis DNA, I've got some goods news and some bad news. The bad news is we're postponing those tests indefinitely. The good news is we've got a much better test for you: Fighting an army of Mantis Men."
Portal 2 writer Erik Wolpaw told Joystiq that the original voice for Johnson was envisioned with a southern accent, but the team decided to go with a midwestern character instead.
You will apparently be hearing "a lot of him," in the game as well, according to Wolpaw.