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.
Satoshi implored people in December 2009 (several months after this post) to postpone mining using GPUs for as long as possible[1].
[1]: https://bitcointalk.org/index.php?topic=12.msg54#msg54