Unfortunately, I think it's pretty obvious that this delay is to implement a shoehorned multiplayer mode that the vast majority of Bioshock players likely don't even want. Yeah, Bioshock 2's was surprisingly decent, but I wouldn't have played it at all if it didn't have trophies associated with it that I needed for the Platinum, and once I had those I didn't feel any compulsion to keep playing it.
The only other possible scenario is that 2K wanted the game pushed back because they knew it would sell in any time of the year, but Dishonored (as a new IP) needed the holiday sales push.
Either way, this really bites. Bioshock Infinite was probably the game I was most looking forward to this year, and it's looked near-finished for at least a year now.