A.V. Club has struck up an interview with Mike Patton, someone who's provided voice acting for VALVe Software (and Portal). Here's a select bit of it:

AVC: You also had a brief appearance in Portal, the Valve game, and there's talk about you perhaps appearing in their game. MP: I think what they did is they took some material that I'd already recorded for Half-Life, maybe it was vice versa. Basically that came out of the same session of work.