IGN swapped words with the woman herself: Overwatch, Announcer, and Homicidal Supercomputer:
In the recording sessions we talked about how passive aggressive GLaDOS is. Eric Wolpaw is the principal writer, and he’s just so funny and clever. And of course I was trying to sound like a computer. In the recording sessions they would play a computer-generated voice for me. I would try to sound like that but then add in the different emotions they wanted. [Valve] would direct me through the sessions. It was really very satisfying work.
Full scoop at IGN.
(Hat tip: Crooked Paul)