It is, since they recently hired SDL developer - and it appears he will be working on it full time, same goes for darkplaces engine creator.
So this is direct help to open source libraries.
They've been hired to work on their respective libraries and publish the code openly? What's Valve getting out of that?
these guys know linux graphics and audio stacks from the inside out. having such people around the office which you can come to, ask pretty much any question and get a meaningful answer is practically priceless IMHO.
Their libraries (especially SDL) are used when games get ported.