She's not quite sure how to feel about all this love and tolerance.
Considering the fact that the other two times we've seen her have been at night, I can only conclude that this is just how her mane looks during the day, when her powers are somewhat diminished by the more dominant energy of the sun.
Princess Luna and My Little Pony: Friendship is Magic are copyright Hasbro and
Edit: I'm overwhelmed with how popular this has become in so short a time.
I wish I could thank everyone who's viewed/commented/faved this, but time just won't allow it. So let me just thank you if you fave this, or even if you just view it and like it. Edit
(5/10/12) Changed the title.