Gamasutra and Microsoft's careers website is seeking out an engineer to work on a "game" that will support up to "300,000 players in real-time." The job ad states that Microsoft is working on "building an extremely high performance system to extend console games to the server in new ways" and that it's "creating a completely new set of web services" for Xbox Live and Xbox.com integration.
It sounds to us like Microsoft is planning to overhaul the Dashboard and Xbox Live service and combine it with an MMO-like system (given the 300K-player support). Or, it could be building a separate virtual world interface like Sony's PlayStation Home. What do you think Microsoft could be working on?