From this month's Game Informer article on Bioshock 2

"Rapture barely survived. A surface-dweller came down almost 10 years ago and struck at the heart and mind of the once beautiful underwater utopia, plunging its inhabitants into the depths of desperation. First he assasinated the city's founder, Andrew Ryan. Next, he killed Frank Fontaine, the man who controlled the Adam. Then with cavalier disregard for the chaos his actions had triggered, the surface-dweller left - and so did the Little Sisters. That's when things in Rapture really got bad. Trapped in a dead man's crumbling dream, the frantic splicers fought savagely over the dwindling Adam supply, killing friends and family to feed an appetite for perfection. Big Daddies continued their lonely patrols with no one to protect...until she showed up. Now, Big Sister has brought the little ones back to Rapture, and the Adam is flowing again. A forgotten lust for the substance has reignited, and the Little Sisters are its gatekeepers. You need the precious sunstance as much as anyone; Adam fuels your plasmids and superhuman strength, but unlike the crazed splicers, you don't need to attack the young girls for a fix. After all, the Little Sisters trust you. You're a Big Daddy. You're Family."

Since you're a renegade Big Daddy, you'll need to take another one out to get a Little Sister, but this time you have the choice to Harvest or Adopt. If you adopt her she rides on your shoulder looking for corpses to harvest Adam from. Letting her harvest turns into an event where a horde of splicers come out of the woodwork to kill your Little Sister and you have to protect her, which they say is easier said than done, but if you succeed the Little Sister shares some of the Adam with you.

Big Sister sounds terrifying. "If you start seperating Little Sisters for Big Daddies - a necessary act to gain Adam - the Big Sister begins to take notice. She might not do anything at first, but once she determines you are a real threat, she hunts you down with a vengeance. No place is safe; she will find you anywhere you hide, and she is a foe beyond your ability to defeat. 'The Big Sister will always be a desperate, desperate fight. She can find you wherever you are, so you have to prepare for the fact that the hardest thing in the game is on the scent.' Players are given some forewarning. With each Big Daddy you kill, the game communicates how close you are to calling down the wrath of Big Sister. Once you cross that threshold, you know she is after you, and the best outcome you can hope for is to choose the battleground and survive. A metallic skittering noise is follwed by an agaonizing scream. In a concerned and sing-song tone, the Little Sister on your shoulder says "Mr. B, Mr. B...Big Sister doesn't want you playing with me..." Then you see a red glowing eye in front of you, and Big Sister springs into motion."
It is then explained how difficult the fight is, and the odds aren't in your favor.

Overall the game sounds pretty badass.