My older daughter attended a fantastic preschool, and on two separate occasions she came home with bites (from a different kid each time). The thing is, at this age, kids get frustrated quickly and there may not even be anything that happened that would make you think, "Oh, well CLEARLY that's why little Betsy bit Mathilda!" Half the time it's something stupid, like Mathilda didn't like Betsy's shoes, or Mathilda needs a nap, or whatever. Bottom line, preschoolers are like little animals. No matter how well trained they may be, ultimately they're unpredictable. No amount of supervision will keep every bite from happening. However, if this same child keeps biting other children, it's likely she'll be asked to leave the school.
The only thing that I found helped was to discuss with my child why the other child bit her. (Once she confessed that she had just hit the other kid on the head! My little angel?! Ack.) We talked about how she needed to use words instead of hitting other kids, and if her friends were not using words, to go tell the teacher. Other than that... keep a pack of frozen peas in your freezer. They work great as an ice pack. =)