Oh dear. This isn't going to go down well. This GamesScoreBlog post's confirmed that Fable II isn't going to ship with online co-op, as promised.
The feature's going to be added at a later date as it's still being "tweaked". From the entry:
We just got word that while we will be able to play the single player and local co-op on day one, they are still making tweaks to the online co-op. Don't worry, it is definitely still coming. Seriously, as a gamer I'm glad they're taking the time to make it right instead of just rushing it out the door. Lionhead and Microsoft Game Studios are looking to get the update out within the first week of launch in North America, so again, don't think this is going to be a decade long wait. I suggest you do what I'm going to do - go play some more pub games and get a ton of gold to buy cool stuff as you start the single player or local co-op game.
Having said that, as you are playing through the game, you will still be able to see your friends as glowing orbs as well as chat and trade with them.
As soon as I get information for other regions and the date that the update is expected, I'll let you know.
PS - Lionhead sends a huge thank you to all of the Fable fans, and asks for your patience and understanding. This is their most ambitious game to date and can't wait to get you playing their vision as soon as they possibly can.
We're saying nothing.