Siri's voice is computer-generated, of course, but the digitized sounds are based on the speech of a real person. Some websites were trying to find out who, eventually prompting Susan Bennett to come forward and admit to being the voice behind Siri. A fascinating article on the CNN website tells her story. She's been the voice of everything from Delta Airlines announcements in airports to the GPS devices. As a professional voice for a company called GM Voices, hers was selected as the voice which would be used in a database of words for constructing speech.
She spent four hours a day for a month recording nonsense phrases. Sound engineers extracted the whole range of sounds in her spoken language, which is then used in synthesizing human-sounding speech, as with Siri. People who knew Susan actually recognized her voice when Siri first appeared. You can hear the similarity in the video that's part of the CNN article.
She admits in the video that it's odd interacting with Siri and hearing her own voice.