Valve has already announced to not expect a sequel to their acclaimed 2007 puzzle shooter Portal this year but apparently they are already thinking about the voice cast for Portal 2. According to Kotaku, casting sheets have gone out (via a subscriber based entertainment industry web site) to voice talent for auditions for Portal 2.The actual voice sessions are due to begin in July.

The story has an extensive character study of one of the new characters in Portal 2 (complete with a drawing/portrait of said character) so if you don't want to learn more about the game's storyline don't go to the provided source link. You have now been given a sufficient spoiler warning