[sokoloff]

Not GP, but it might have been this one of Tony presenting:

at Startup School 2009: https://www.youtube.com/watch?v=Nlop3TwaMJM (poor AV quality, unfortunately)

but seems most likely to be this one (which includes the offer for the culture book) Web 2.0 Summit 2008: https://www.youtube.com/watch?v=JQ2DmNk3YjQ (good AV)