Scott,
I use some air hardening steels occaisionally, and send them to Paul. You can do a Google search for Paul Bos Heat Treating and his page at the Buck knives site will come up, with all of the needed info. He charges a flat 75$ for up to 20 blades (I think), or 15$ per blade for small orders. He is widely considered the best in the business of heat treating air hardening steels, so I use him. I read somewhere, maybe on his site, that no blade ever leaves his shop warped, which makes me comfortable grinding blades pretty thin before sending them to him. Some makers even etch Paul Bos HT on their blades to let customers know that Bos had done the heat treating. I don't think you can go wrong going that route.
Knives 2005 ( I don't have 2006 yet) lists 12 commercial heat treaters, other than Bos. Texas Knifemakers Supply , as Mike said, offer the service, and I think they are pretty cheap, but I can't speak to their quality, one way or the other.
Todd