Thanks to everyone who commented & emailed me on this one. Turns out that T-Mobile simply changed the IP addresses on some of their servers, and T-Zones Web access was not cut off after all. Switching the addresses - most easily accomplished by way of an upgrade to Big Boss' T-Zones Hack (available via Cydia) - did the trick. Web access via T-Zones is restored and all is well again in the land of cheapskate iPhone users. Huzzah!