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!
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