it was in an interview w/ Herbstreit
back a ways.
They absolutely do Gameday for non-conference September games. See Ohio st vs SC last year...
The conversation came up with the Cal fans, ironically enough, in their hopes of landing gameday for their Tennessee tilt a few years back. someone had submitted the question, and Herbie actually addressed it on the air at some point.
Now, the rules might have changed along the line, but I do remember him specifically referencing ND as the one exemption. I've seen GameDay live at ND, the RoseBowl, and at the LA Coliseum a handful of times. It's alot of fun, the students get pretty jacked for it. It would be great if they came out for a game next year (Air Force, Cal, or Wisconsin)...but at LEAST for the 2010 SC game.