The makers of such acclaimed single-player video games as Portal and Half-Life 2 want all of their future games to support connected, non-solo gaming, in some way, at all levels.

That assertion first appeared in the Final Hours of Portal 2, journalist Geoff Keighley's recent behind-the-scenes chronicle about Valve's newest game. It's an assertion he told me he heard directly from Valve founder Gabe Newell and the company's project manager Erik Johnson.

"Portal 2 will probably be Valve's last game with an isolated single-player experience," Keighley wrote in Final Hours, "What this all means is something Newell is still trying to figure out."

Keighley told me that he considered the comment "curious," noting that the quality of the solo-only main campaign of Portal 2 was a fantastic piece of work. (I've checked with Valve on this, but they didn't reply by press time.) The signs that solo-only modes are on their way are there, not just from within Valve but all around the pioneering games company.

Valve's Portal 2 introduced multiplayer to the Portal games through a two-player co-op mode. The company's recent 2008 and 2009 Left4Dead games were presented as a primarily-multiplayer experience, even on consoles where such an animal is about as rare as a Nintendo-made Halo game. Valve has also continued to aggressively support its multiplayer Team Fortress 2, a game launched alongside Portal in 2007.

The company's primary vehicle for single-player-only experiences has been the one that the public hasn't seen anything new of since 2007. The campaign portions of Half-Life put players in control of hero Gordon Freeman; other players haven't been able to join the game's main adventure. While Valve has used its Half-Life games to present a more lively, less lonely first-person-shooter campaign, it has done so strictly through improving the artificial intelligence and acting of Freeman's computer-controlled allies, namely Alyx Vance (pictured with our hero above).

Outside of Valve single-player-only games have been vanishing. After a long stretch as leading single-player franchise, Super Mario games on consoles now include second-player support. Series that launch as solo-only such as Uncharted or BioShock add multiplayer for their second installment. Hold-outs like God of War seem destined to add support for multiple players, somehow, some way. A company like Capcom doesn't just make millions with its four-player series Monster Hunter, but it's slowly but surely been pushing its formerly single-player-only series, Resident Evil, into a vehicle for multiplayer console Resident Evil games (the upcoming co-op and competitive Operation Raccoon City) and multiplayer portable ones (the co-op Resident Evil: The Mercenaries 3D). Even Capcom's Dragon's Dogma, though single-player, simulates a multiplayer experience by giving the main player a host of computer-controlled allies who behave as joining Monster Hunter gamers might.

The comment from Valve is striking, though, in that it doesn't sound like Newell and Johnson said they'd probably never ship a game that didn't have a multiplayer mode somewhere in it. Rather, they told Keighley they "probably" wouldn't make a game "with an isolated single-player experience." That would mean no more modes that couldn't connect in some fashion to other people. Would, say, letting a second player control Alyx in Half Life 3 do the trick? Or could Valve be cooking up something less expected?