@Josh:
Most of the gaming which is done on Linux is done through Wine or some form of Wine anyways.
Of course it is.
But Steam isn't about porting so much as
content delivery, so find some developers willing to use cross-platform engines (see
Unigine, ferinstance) or bundle games that work well in Wine and I guarantee you've got customers.
Besides, everyone I've talked to about this has told me the Mac platform has never been a platform of choice for games, so it's just as mysterious to them why Valve would build an official Mac port that's getting all kinds of fanfare at the moment.
So the question remains, why
wouldn't they build Steam for Linux, especially when so much work has already been done on it?
As far as market share, read on...
@Zaine:
At what point does gaming have to be
sufficiently open source for Linux users to magically become a gaming force?
Of course proprietary code is always going to be a sticky point for the philosophy-conscientious among the Linux crowd, but you develop Linux ports of quality, popular games and I bet some
exceptions will be made.
Besides, nobody said you
couldn't have proprietary software on your Linux box, did they?
As far as numbers are concerned, money to be made and the market thereof, simply read here how 2DBoy and Frictional made out with their games:
http://jeffhoogland.blogspot.com/2010/05/why-there-is-market-for-linux-games.html
What was that?
10-12% of total profits from a platform that only has
1% of the desktop market?
When asked to donate, Linux users donated almost
twice the average?
No market for Linux gaming? 
Nuts to
both of you...
(ahem... the fresh-roasted kind with an even ratio of peanuts to exotics
)
