Right now most games that have "online" play use one computer as the host and a limited number of players can log into the arena. Most environments provided in this way are "non-persistant" in that no character or environment data is saved when the game is shut down.

Even your largest MMORPG (WOW ect) cuts its games into "shards", basically importing a character from a primary storage server and updating its information on its chosen world. Each shard is usually a bank of servers that provide a persistant environment for a continually updated character to play in. As of yet there is no environment built to give more than a static number of players (even the largest WOW shard can only carry a limited number of players)

There are LAN setups that utilize shared processor speed. The old classic RTS Total Annihalation utilized shared processor speed through some voodoo to create a faster, stronger computer opponent. Also software has been developed to link several computers together to act as one large "virtual" server.

That being said processor power is still best handled by a central server, or group of servers, in the same geographic location. The lag caused by 100bT connections to the internet make shared processor speed in a persistent online environment problimatic at best.

You also have to take into account the fact that the MMOFPS genre is still relatively new and imperfect to the gaming companies all the main problems that Taiko stated about naming something a MMOFPS is the reason company's are shunning away from putting out the bucks to back up the games with the needed hardware.