Miaghstir: Not too many existing games have been ported to the Mac since Steam:Mac was released either, have they? Valve's source games, sure, some indies, and a handful others through Cedega/Wine. I'd wager the Linux ports will be pretty much the same ones.
dirtyharry50: No, the Mac does get some native releases such as Civilization V, Two Worlds II, King's Bounty, Torchlight, GTA games and others and outside the world of Steam a couple noteworthy ones I can think of include Company of Heroes, World of Warcraft and the upcoming Diablo III. Also, there is nothing wrong with getting something like The Witcher bundled up with WINE as a commercial release. Who cares how it is ported as long as it works well? Mac is ahead of Linux in getting native versions of games like the ones just mentioned. But Linux may be growing enough now to start getting more attention as well like we're seeing here with Valve jumping in.
During the early days of WINE-based porting of games to OS X, there were some truly horrible releases. X3: Terran Conflict was the prime example of this; the fact that the game was naturally horribly buggy to begin with didn't help matters. C&C3 was stable (at least), but poorly optimized. The only exception was HoMMV, which was actually tuned to perform better on lower-end hardware.
With WINE having matured a lot in the past few years now, that's essentially a non-issue, as a lot of games work great on WINE/CrossOver on OS X. Frankly, I don't care how a game is ported over to Linux/OS X, as long as it performs well and has feature parity with the native Windows version. Native ports would be preferable, in theory, but unless a game is cross platform by design porting takes a huge amount of time and resources.
It'll be interesting to see the general reaction of the Linux community as a whole to have Steam (and potentially all SteamPlay titles) on Linux; the most popular DRM-based computer game digital distribution platform on the planet, on an OS whose platform was based and is centered around a philosophy that is fundamentally against DRM...it's going to be interesting to watch what will happen.