

Not sure who the mystery girl is, but there is speculation that it's Shannon Woodward - she was at Katy Perry's birthday dinner, which Rob also attended. (Why, I have no idea... but don't worry, I'm not judging you, Rob. That I Kissed a Girl song is unbearably catchy. But have you heard the Corin Joel version? So much better!)
UPDATE: Yes, that is Shannon Woodward. She is apparently dating Rob's friend, Andrew Garfield.
[source]