Today I saw her in a behind the scenes clip that was released at SDCC. It was SO quick I didn't even realize it the first time I watched it.. then later I was thinking about how 'Kate' was in the movie and I thought I remembered seeing a female elf in the clip... So I went back and there it was; about 20 frames of an elf and YEP, it's her.
It's at about 11:38 on this clip:
The Hobbit - Production Video #8 (2012) Peter Jackson Movie HD - YouTube
Someone told me there were more images of her at Comic Con, and I would love to see them if anyone has them. That costume is really cute.