A post over on the Sega UK site reveals that the multiplayer for Empire: Total War will be released after the main game, free to purchasers of the main game. There will be a testing period and "an opportunity to participate in the 1 versus 1 multiplayer campaign mode beta will be made available to all Empire Total War buyers post release."
A multiplayer campaign mode. Interesting. This might well be related to the one month delay of the game: it will now appear in March.