G4TV writes:
"Halo 2 is the reason you can't have more than 100 friends on Xbox Live, I reported earlier this week. Technically, it's because the old Xbox Live tech, which Halo 2 runs on, can't support more than 100 friends, but Halo 2 remains the only viable community preventing Microsoft from shutting down the previous XBL. Microsoft has since issued an official response to my story that provides little clarity."