Cindi, I think you have gotten a great range of answers across the board. I'd approach it first, as Bridgette commented, by taking the position of asking if there is something you have done- and how can we make this work bettr for everyone. Let the agent know, however, that you really can't have negative comments made to your buyers about your professionalism, and that you are going to be here "to help through the whole transaction" whether or not the builder expects that.
Then, if the other agent still seems condescending, uncooperative or negative you need to go to whoever is above them and let them know it is happening. Keep each conversation along the way, including the past comments in a log, so that you have documentation for your broker and the other agent's broker if needed.
As Michelle stated, some people are just jerks! But, in my opinion, we- as professionals need to see to it that the level of professionalism is kept up as high as possible- so we can't just allow the jerks, "ethically challenged, etc just continue "willy nilly" without commanding respect for ourselves.
Good luck. If you ever have clients with needs in the North San Diego area, please give me a call. I'll make sure YOU look like a STAR by taking great care of them!
2007 President, North San Diego County Chapter Women's Council of Realtors