While I was only earlier interested to Steam going onto the Mac because of the community features (Chat, community/groups, ...) that would be available soon, I have to agree with the statement above.
Cider Ports apparently run slow and are inefficient, according to some people, and I find that my Mac runs games better in Mac than PC (When it's native.)
Does anyone think that upcoming games such as R.U.S.E. or the currently released Assassin's Creed 2 will make it on the Mac as a native game?
No....
Some of you sound confused.
Realize, while Steam is coming to Mac, it doesn't automatically convert all its games to work on Mac. Steam is just content delivery.
What I'm saying is, say Steam came out today. You won't be able to download any game you want, much less have it work on your Mac. You will only have access to Mac games.
Valve has confirmed all of it's games (read, Valve made games) will be released when Steam releases. That means HL2, L4D, etc
R.U.S.E and others will not be available to you on Mac. They will not be emulated, downloadable, or anything. Steam.app does not magically convert Windows games to Mac games.
That said, Valve's games will be native ports. Not Cider, no emulation. Fully native.
Other games might be available at launch, or at a later date, depending on the developer. Other games means games that have Mac versions. Such as Machinarium, Plants vs Zombies, etc