I think it's a cute, youthful extension of the fur trend but the anime community has been putting animal-like these on top of their heads for far longer. Exhibit A to the left was taken over 8 months ago.
I got a plaid, bunny-ears hat and a blue kitty cap from a convention about 2 years ago. My mom is embarrassed to be seen near me with the bunny hat so I think she's hidden it.
Pictures from High Snobette, Trend Hunter, DesignScene.net & of course, Trend de la Creme.