The guys at Phoronix have some inside info that gives more credibility to the rumors that have been circulating. They don't go into the information, or (obviously) the source, so take it as you will, BUT... Phoronix does have a good track record with such things...

"Valve Software has yet to officially announce Linux clients for any of its software, but at Phoronix we have received information confirming that Valve is indeed porting its very popular Source engine to the Linux platform."