It's here. NVidia and ATI have released some nasty fast video cards that, if someone can figure out how to program them for general computation use, will offer an amazing amount of power. I'm sure there's a grad student somewhere right now writing up a thesis proposal to code it.
http://www.wired.com/techbiz/it/news/2008/06/gpu_power
http://www.wired.com/techbiz/it/news/2008/06/gpu_power