Hi G..
This is a tough situation. Regardless of all else that changes in the lives of the kids, there are ALWAYS bullies.
I've never been bullied myself, but have stood up for others. And, my Dad told me pretty much the same thing Mindy's mother told her. Let them make the first move, then "clean their clock" and if I didn't, then I'd get it at home. I know that sounds harsh. But, if you don't allow your child the ability to defend themselves, you kinda set them up. Also, does the bully threaten your daughter when her (your daughter) friends are around? Witnesses are awesome things!
I understand what the other lady said about getting in trouble. I understand the set up for a lawsuit (we are a sue happy nation!), but the bigger issue here is your daughter. She has to know that she is allowed to protect herself. Armed with that knowledge, she can then throw down her "bring it on" with the understanding that you back her. Bullies get away with being bullies because people are afraid to confront them. And, mostly, they're just hot air.
You may want to throw a "heads up" at the principal. Just let him know that your daughter is having issues with a bully. That's all - no names. I know people say talk to the principal and the bully's folks. But, I've known a lot of people who did that and the bullying just got worse. You've got to weigh what you know about this kid. If you don't know much, then you could ask your daughter. But, all in all - gotta tell you I think you did the right thing. But, if it does come down to brass tacks, be there to support her thru the aftermath.
Good luck. I hope it IS nothing more than hot air!