Microsoft's official marketing blog has confirmed this morning that the patch to enable Fable II's online co-op features has been certified as something that (probably) won't make your Xbox 360 explode or speak in tongues. This means that by the time you're able to properly buy Fable II tomorrow (or on the 24th if you're in Europe), the online co-op should be ready to go.
However, a time for the patch wasn't announced, so I guess it's possible that if you ran right out first thing in the morning and got the game, it might not be up yet.
Still totally against day one patches over here, but considering you'd need online access both to use and to download the patch, this case doesn't seem like a huge deal.