Whilst BioShock 2 did have a tacked on multiplayer component, it's predecessor, which was developed by Ken Levine's Irrational Games studio, did not. Ken Levine argued in a recent episode of GTTV, that multiplayer was not right for that experience, and went on to say that if it isn't right for BioShock Infinite, then it will not be included.
"We've been experimenting with this [multiplayer], even during BioShock 1, and the question for us is always, 'do we have something that really stands on it's own, as an equal partner in the experience, not just a tacked on [experience] for the marketing department?'"
Levine continued, "We won that argument in BioShock 1...and if we think we have something that's a valuable part of the experience that stands tall, but at this stage we haven't determined that."
So it looks like we might not see multiplayer in the third title in this franchise.