Actually the first person known to GPU-mine (supposedly an Apple OS engineer, friend of "BitcoinEXpress" on bitcointalk.org) claims he did it in May 2010, wrote his own miner, running on 3 x AMD Radeon HD 4850 graphics cards. The claim sounds plausible. There was a small bump in the global hashrate in May that seems to correspond to the power of this setup.

I commented on it here: https://bitcointalk.org/index.php?topic=40876.msg520645#msg5...