A total of six multiplayer maps from Halo: Combat Evolved will be playable in Halo: Reach's online multiplayer along with Halo 2's "Damnation." Halo: CE Anniversary will not support Xbox Live multiplayer whatsoever (outside of co-op). These maps are for loading into Halo: Reach (think: ODST), according to comments made by 343 Industries' Frank O'Connor on Spike TV. He also said it will be a budget-priced game.
Pretty much Halo CE will come with two disks, one with the Halo CE campaign and one with Halo Reach multiplayer with classic Halo maps.