The game is scheduled to appear on PC, Mac, and Linux (as well as downloadable on consoles) in 2008, and will feature a robust character creation system, and all of the Penny Arcade characters. The "Episode One" part also hints at what might be an interesting mechanic-- you'll be able to create a character in the first game, and then carry that character over other episode releases. Joystiq reports that the game looks good, and should be a fun, if short, experience.
So look for an interactive Penny Arcade on your Mac next year. And in the meantime, watch out for those Claw Shrimp. They're vicious.