A back catalogue of games will be irrelevant to the audience Valve is going to market this thing to. They would not be moving forward without publisher support either, so it's probably safe to say Valve has devised a way to port games very easily from a Windows platform to a Linux one, as indicated pretty clearly in one of their Linux blog posts. Apparently it is extremely easy to do with the work they've done, so it would be minimal effort to release Linux versions.
I'm sure they haven't totally forgotten about the back catalogue either. If not available from the start, I expect them to have a pretty sophisticated compatibility layer like WINE, but actually usable and available at some point later. Any performance hit from not having these games be native Linux applications would be offset by better hardware, which we're likely to already be at.
Here's a video of Crysis 2 in Ubuntu 10.10 using WINE. There are also several more games with this that look great overall, like Skyrim. I don't expect Valve to use WINE exactly, but just so people know it can be done:
Crysis 2 (doesn't explicitly show it's in Linux, but I hear it does work pretty well)
Skyrim (explicitly shows it running in Ubuntu)
There are likely issues for sure, but it's a proof of concept more than anything. If something was developed
seriously for graphics intensive games, you could probably see the entire back catalogue no porting necessary.