All we know of right now is a PC release. Whether or not there will be Linux, Mac, or console ports is unknown at this time. Gray Matter doesn't have a website, but Activision, the publisher, has an official Return to Castle Wolfenstein site . Under the name Xatrix, the guys from Gray matter made Redneck Rampage and Kingpin. It's Gray Matter.
Software is involved somehow.
That's not a question. RTCW uses the Quake 3 engine. You've seen it used in games like Quake 3 Arena, FAKK2, Star Trek Voyager Elite Force, and Alice. There will indeed be multiplayer shipping with the game. It has been consistently stressed, however, that this is primarily a single-player game.