According to Forbes, Final Fantasy XI is indeed coming to the Xbox. The article does not go into any detail on the specifics, so it is unclear whether the original Xbox or Xbox 360 is the intended platform. The Final Fantasy series of RPGs has been a stalwart for Japanese console makers, Sony in specific over the past iterations of the PlayStation.
UPDATE: Reports from those viewing the live stream indicate that FFXI will be coming to the Xbox 360 and will support network play between PC and Xbox platforms.