One of the first people I recruited for Valve when they were looking for good Linux referrals was Forest Hale, or better known within Internet communities (including the Phoronix Forums and Phoronix IRC) as LordHavoc. He was the lead developer on the DarkPlaces engine, which is the Quake-derived engine that was used by the open-source Nexuiz game and is now used by Xonotic as well. As can be seen when firing up the old Nexuiz or when running Xonotic, DarkPlaces is both technologically and visually impressive, especially for being a non-commercial GPL-based engine. Under contract he additionally was the lead on the Mac OS X and Linux ports of Quake Live. He's also done other Linux contract work, but for the work on the open-source DarkPlaces engine is where he's arguably most known.
They are not exactly 'gearing up', but clearly they are working towards Linux.
Over the weekend an email was sent in by a Phoronix reader, Joe Davinson, about an email exchange he had with Gabe Newell of Valve. Below is the email screenshot where he says "Yes" to "Will the Linux version of the Steam client be released to the public before the end of this year?"