Had same problem with Chevron, and I had set write permission to the hosts file, reformatted SDHC card (that was working fine) with SD Formatter tool, re-flashed WP7 fresh.. to no avail.. until I went into WP7 "settings" app > "about" > "reset your phone".
Once I did this, I had no problem connecting to Zune, and Chevron. There must be a flag that gets set by the reset activity... Hope this solves your problem too!
Retired: ATT Tilt (Kaiser), ATT Fuze (Raph110), HTC HD2 (Leo - T-Mo US), Samsung Galaxy S 4G, Samsung Galaxy S II Hercules, Samsung Galaxy S III
Nook Color - Rooted & OC 1.2 Ghz
ATT Samsung Galaxy S 4 - awaiting MF3 solutions