It was already revealed in an interview last night (story) but id Software and Activision have formally announced today that Nerve Software has been contracted to create the multiplayer component of Return to Castle Wolfenstein. The press release doesn't offer any new details beyond this general description of what we can expect:

In addition to an action-packed story-driven single-player experience where players must infiltrate deep into the heart of the Third Reich, Return to Castle Wolfenstein's multiplayer mode enlists gamers to hone their tactical skills in a series of combat scenarios. Gamers will join the fight as an axis or ally soldier and be challenged to intense, realistic campaigns like storming beachheads, infiltrating military installations, holding strategic positions, and more. The game features multiple objectives spanning a variety of maps, including an Axis airbase, a war-torn German village and a secret Nazi laboratory.