I had the same problem as before – Activesync changing my Proxy settings in my Jasjam to use the work proxy, breaking web browsing on the thing via my Telco.
Activesync is different to the Mobile Device Thingy on Vista and although the fix was the same, it took me a bit to find.
Think of this as a Vista Version of KB915151
As below
Go to “Windows Mobile Device Centre”
Mobile Device Settings – More
Connection Settings
This computer is connected to: Work Network (from Automatic)
This worked perfectlly for me as well.
I am running Vista Business with a Samsung i780 (Mobile 6 Pro)
Thanks