My heart aches for your daughter! I strongly agree with setting up a teacher conference ASAP. If the teacher is made aware of the situation, she may be able to help steer other, kinder, children toward your daughter as playmates (ah,the power of suggestion!). I know I've seen my daughter's teachers do this in Kinder and First grade when someone seemed to be left out. The teacher may also be able to talk to the class as a whole about how they are all friends and classmates. Again, with an excellent teacher this is often their expectation for the class...that they all treat each other with respect and try to get along. It really can be done! Maybe the "mean" little girl can be taught at an early age to be kind to others rather than cliquish.
Since your daughter has been happy otherwise so far, I'd hesitate to move her at the first sign of conflict. Also, maybe you can encourage her to reach out to the "mean" little girl, or to other kids in her class. Some kids really just don't get along (we experienced this with my daughter in her first preschool).
Best wishes for a peaceful and swift resolution for you both!