Actually at least RunningWithScissors who are currently making Postal 3 are very well working on a Source client for Linux. I asked them whether there would be an actual Linux Source engine and other Source titles for the platform and their reply was:
"we are definately working towards this! keep in touch!"
Postal 3 is said to be launched with a working Source Linux engine and I can't think of any way Valve WOULDN'T put Steam in there. In the end Steam is the easiest part of porting those games.
Considering that Vince Desi's not one to make statements he can't back up, I'm thinking the odds are even right at the moment that we're going to see at least Postal3 on Linux, leaving it open for other Source ports to be possible.
It wasn't Mr. Desiderio btw. it was Mike Jaret-Schachter who told me this but it only confirms what earlier interviews had already promised.