Last season President Palmer offered Nina a plea bargin in exchange for information about the bomb.
That's why she's free as a bird.
i thought that went bad, when she tried to kill Jack and hld him hostage. after the sniper shoots her, she is taken back into custody and jack whispers something in her ear....i didn't think it mentioned her again last season.
We must have missed something...
Amazingly Nina planned and executed her escape within the scope of 24 hours...
EDIT: Sorry for the bad spelling. I'm dictating to Ali Sama.