I never did manage to figure this one out. The registry key HKLM\Software\Microsoft\RTC\Transport\QoSEnabled=1 never seemed to take effect on x64 versions of Vista or Windows 7. Wireshark packet traces would show no tags, but x86 clients worked just fine. Turns out the registry key you need to set for this to work is under the WoW6432Node which makes a ton of sense after seeing it. The DWORD you need to set is this one:
Thanks to Matt McGillen for posting this originally.