I consider myself an avid gamer, and have been so since the 80s.. I haven't had a windows box in a dozen years.. When I see it, people using it, or god-forbid, have to use it myself, i'm reminded WHY i don't use windows..
I've been playing TF2 since it came out, under linux, no problems..
No, it's not better than the native awesome it could be if ported.. (hell, it runs on Mac now, half the job's already done)
I realize I'm in the vast minority of people who consider themselves a "gamer" while using linux exclusively.. but we wouldn't be so rare if Valve had the balls to actually produce a linux client.
As for that fraction of linux users who are all free software uber alles; they're the reason the term "open source" was coined. While Free Software and Open Source Software philosophies differ, Open Source Software's philosophy that "it makes sense for some software to be paid for", and that more penguins in general would be inclined to buy games for linux anyway, is all the more reason to make Steam available for linux..
but whatever.. it hurts just the same. =(