Yes, we already know that every 360 game must support Live Aware, because Xbox live seems to be the very heart of the next gen sucsessor.But rumor around the internet suggest that Fable 2 will support some sort of MMO feature. It is unknown yet wether or not it will cost an additional fee on top of the Live subscription

(example: Phantasy Star Episode I & II for the Xbox) or will simply run free.