Awesome, I hope it works with open source drivers.
As much as I hope this as well, it's highly unlikely, because the max the Opensource drivers support is OpenGL 3.0 as far as I know. Serious Sam 3 looks like it uses a lot of very advanced shaders, probably around the level of OpenGL 4.0 or higher
Looking forward to this. Haven't played a good, fun twitch-FPS since Doom & Quake days. I'll probably check with the Phoronix crowd after release to see how it runs before I purchase though - I've seen some comments regarding a "broken Mac port" in some forums. While I trust that Valve would continue to improve their engine and games on Linux post-release, I'm not so sure if Croteam would just do a 'dump and release' or continue to provide patches for the game afterwards.
This is all very good for Linux but I'd rather see a Call of Duty client instead. Until gaming companies get their heads out of Microsoft's rear end then don't hold your breath for any mainsteam games being developed or ported to Linux.