I have similar issues with WMDC, the Vista version of ActiveSync. There's an informative thread on this site where I've documented my issue and have linked other threads that had various solutions:
http://www.windowsmobiletraining.com...px?PageIndex=4
I originally had no problems synching the phone, then all of a sudden the PC stopped recognizing the TP. I've tried almost every method to remedy this situation, but none of them thus far have worked.