It's definitely tucked into her shirt. You can even see a hint of her ends peeking out in the key-hole at her shoulder there.
You know Kim doesn't go in front of photographers without checking a mirror first—plus, Kanye was with her, and you know he would have told her if there was something about her look that wasn't "done" in his eyes. Which leaves me with the conclusion that this was a hairstyle choice. Interesting, no?
What do you think of this look? Silly or kinda cool?