I think valve's core games on linux will be there at the same time the linux steam client arrives to give it that push. It was that way with the mac. However, I think there will still be some time before we start seeing the majority of games ported to the three platforms.
Sometimes, mac ports that are done by third parties are not on steam because they can't split the revenue, so you still have to get the mac version separately.
But even with all the hurdles, I think this is a great step in the right direction.
I think only a fraction of existing games will be ported. However, now there will be a much stronger incentive to use cross-platform engines in the future. It used to be "windows + mac port" for a lot of games. "windows + mac port + linux port" sounds silly, so the obvious solution is to start with something that already is cross-platform.
Wasteland 2 chose Unity engine because it's so cross-platform. It even has unpolished Linux support. The Linux port of Unity hasn't been released, because developers were doing that in their spare time and it wasn't up to Unity creators' standards. Now, InExile has been given Unity source for purposes of creating a decent Linux port. It's a single anecdote, but it illustrates what I'm saying above. Makers of Unity will probably want to get the most out of it, so Linux port of Unity will be contributed "upstream" and it's very likely that Unity will support Linux from now on.
Most Source engine games run perfectly well under Wine already. I think a Linux port should be pretty simple at this point. As for "Windows + mac port + linux port", Portal 2 was launched on PS3, so the code is pretty portable.
Source actually uses Direct3D rendering on Microsoft Windows PCs, Xbox and Xbox 360, and OpenGL rendering on Mac OS X and the PlayStation 3. It's very portable.
Often, the difficulty of bringing a game to OSX is porting DirectX graphics code to OpenGL. Once that's done, the hard part of Linux support is dealing with distribution fragmentation and dependencies.
Steam on Linux will probably ship with the audio/graphics/input libraries it needs, so 3rd party developers don't have to worry about environment nearly as much.
Porting is not really so bad issue, problem is that all sound and input port code will not work after year or two because of libraries changing, forgotten and deprecated. What I really do hope that Steam will choose libraries which might concentrate development effort around those. That would be good thing for everyone, not just for game developers.
Sometimes, mac ports that are done by third parties are not on steam because they can't split the revenue, so you still have to get the mac version separately.
But even with all the hurdles, I think this is a great step in the right direction.