Yeah, you read that right. 300,000 players. A job listing issued by Microsoft's says exactly that, indicating that the game will be for the 360 and that it will be in "real-time".
The job is specifically looking for somebody who is "an experienced server developer to help develop our server architecture. We are building an extremely high performance system to extend console games to the server in new ways; creating a completely new set of web services to support dynamic programming information of games and scheduling data".
The successful applicant and their team will be responsible for "building a backend capable of hosting a 300,000 player game in real-time with real money on the line (anti-cheating, etc)", along with constructing a new "gameplay scheduler" for Xbox Live, which sounds like it would allow players to formally organise playing sessions for this game (and others as well?).
What title could possibly require 300,000 players? Or even hope to attract that many?
Hrm. Hmm. How about that fancy Halo project?