In the words of a post on the Fable II Dev Blog:
Remember a couple of days ago (or was it a week) where on a blog it was mentioned that Fable II would be shipping 'without' Online Co-op? And remember we said we were busting our balls to get it ready in time for a 'Day One' release? Well--Good news folks! It looks like we've done it...for all of you who are connected to Xbox Live, when starting up your copy of Fable II.
The update will be available upon inserting the Fable II disc, "taken into account we don't fail certification!" jokes the blog's author.