The GPU miners are widely used nowadays, but that wasn't always the case. When was the first GPU mining software made available publicly?
This post from SmokesTooMuch seems to be the first time GPU mining was suggested:
Since the coins are generated faster on fast machines, many people will want to use their GPU power to do this, too.
So, my suggestion is to implement a GPU-computing support using ATI Stream and Nvidia CUDA.
Here's satoshi in December 2009 asking for people not to use GPUs to mine:
We should have a gentleman's agreement to postpone the GPU arms race as long as we can for the good of the network. It's much easer to get new users up to speed if they don't have to worry about GPU drivers and compatibility. It's nice how anyone with just a CPU can compete fairly equally right now.
So it seems the idea was around for a long time before the savages broke the gentleman's agreement. :)
According to the History page, puddinpop's CUDA miner was released September 18, 2010. The article also shows that the first public OpenCL miner was released on October 1, 2010.
Prior to there being publicly available software, the article shows July 18, 2010 as the date that the first block was mined by the first person believed to have been using GPUs. Art Forz developed and used privately GPU miner code and had built an OpenCL GPU cluster for mining.