An SEO Chat thread named Who is GoogleGuy? offers some theory in who the mystical GoogleGuy is who travels the SEO forums.
Some suggest that GoogleGuy is really not just one guy, but rather several Google employees. The thread creator suggests that it might be one of the following; matt cutts or larry page or a 'do no evil' engineer or possibly a spy.
One member links to a Google Blog entry named Greets from GoogleGuy! which gives some insight into GoogleGuy(s)'s personality.