In the IGN interview on Monday, Frankie said that dedicated servers were the answer. As I understand it, Halo CE didn't have the hooks to easily turn Xboxes into hostboxes over XBL. Since the games would need to be hosted on a player's Xbox, lots of code would need to be written to make that possible.

With a dedicated server, though, those problems are no longer a concern. The games do not need to be hosted on a player's Xbox. Instead, it's something akin to system-link play...only every player's Xbox is routed through a dedicated server instead.

That's my layman's understanding of it, anyway.