Corran Horn from neogaf.com tells us some of the features of Zipper’s MAG. Some highlights:
All audio cues (planes flying by, air raid sirens) are actual players causing it or player triggered actions….

The first map pack, called “Steel and Titanium” will feature two maps, one set on the ISA tech cruiser “New Sun” and featuring more vertical gameplay, and one aboard two Helghast freight trains that can be leapt between, if you’re willing to endanger yourself with all the environmental hazards around.

This new gameplay footage shows a general combat sequence, with the player very obviously taking on the role of a Big Daddy. BioShock 2 is set to release before the end of the year.