Red Orchestra: Ostfront 41-45 was released for Windows back in 2006 and runs off the older Linux-compatible Unreal Engine 2.5 (not the newer Unreal Engine 3), but to date we have not seen any Linux client of this game developed by Tripwire Interactive that takes place during World War II. Ryan clarified that he was contracted to just perform the Linux server port and then for Mac OS X he did the server port too, and he is also doing a Mac OS X client port for this game and that version will soon be released.
However, once Valve Software releases the Steam client for Linux, it looks like we will "almost certainly" get a Linux client for Red Orchestra: Ostfront 41-45. Tripwire doesn't use the Source Engine for this game, but it does use the Steam delivery software for distribution.
Not only would Red Orchestra: Ostfront 41-45 likely come to the Linux gaming land, but so would Killing Floor. Killing Floor is a mod for Unreal Tournament 2004, which of course already has a mature Linux client, that is developed by Shatterline Productions and distributed via Steam.
Below is the excerpt from the email inquiring about the Red Orchestra Linux support and then Ryan's response.
Tripwire Interactive, http://www.tripwireinteractive.com/
* Contracted to port Red Orchestra: Ostfront 41-45 to Linux and Mac OS X.
I can only assume you meant to say "server" somewhere in there, although I hope I am wrong. I would not be surprised if I am wrong though that you would not be able to talk about it.
I abbreviated to fit the line in 80 characters. It's the dedicated server for Linux, and client+server for Mac OS X. The Linux server has been shipping for years, the Mac port is in progress right now (shipping soon). If there were a port of Steam for Linux, we'd almost certainly do a Linux client, too, but at the moment there isn't one outside of those poking around the Steam binaries on the Phoronix forums, so we haven't even started looking at a Linux client.
This is true for Red Orchestra and Killing Floor (KF is now shipping for Mac OS X, though...we figured we'd work out the bugs and move on to RO from there).
For the sake of it not being original research on Wikipedia:
(Despite the word "rumor," the article says John Gibson from Tripwire confirmed this...which is true: he did.)
So beyond Valve's Linux move causing Source Engine games coming to Linux like Counter-Strike: Source and the Half-Life 2 series (along with other non-Valve titles like Postal III), it looks like independent game developers that simply rely upon Steam for digital distribution may be more prone to offering up Linux clients once this Steam client is available to provide an easy purchasing and digital distribution platform.
While some find it hard to believe right now, we have confirmed that Steam and Source are coming to Linux and the fact that there is an early Steam Linux client binary. The UK's Telegraph has also independently confirmed this information too, as was confirmed by a reader's inquiry to the publication.
Too bad Ryan didn't comment on the Unreal Tournament 3 Linux situation within this email.