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.