The man who bought pizzas with bitcoins stopped such trade because he was no longer able to mine thousands of bitcoin per day.
He opened for pizzas x bitcoins trade in May 2010 but stopped it in August 2010, just three months later. If we check Bitcoin network hashrate and difficulty, I believe there was sharp increase within three months from May to August 2010.
Well I didn't expect this to be so popular but I can't really afford to keep doing it since I can't generate thousands of coins a day anymore 
Thanks to everyone who bought me pizza already but I'm kind of holding off on doing any more of these for now.