I gave up since TAC couldn't reproduce the problem. It sounded like they had to build a Vista machine since they didn't have any for testing. I tried two clean builds as VMs and Communicator worked fine. I haven't been able to identify what breaks it...
You should open a TAC incident. I've been working with them on one since Friday. I have verified that the configs are getting to the phone correctly. Ethereal shows that the app is not even attempting an HTTP request, so something in the app or OS is...
Is anyone running 2.1 on Vista yet? I tried on 2 different Vista laptops. The same softphone device works on non-Vista builds fine. The Vista clients will register and the phone functions work, but they cannot access web services such as directories ...