According to the Steam news page, BioShock 2 was updated today with some fairly significant changes. First and foremost, it seems as though the requirement for Games for Windows Live has been removed.

In addition, the much loved Minerva's Den DLC has been added, for free (if you own the game already), to the Steam version of the game. That's not all, though.

If you buy BioShock 2 on Steam after today, you will be able to purchase Minerva's Den separately for $5-$10 (USD) depending on if it's on sale or not.