So my iPhone data went out again about two weeks ago. For the uninitiated, I use an unlocked iPhone on T-Mobile and get data via the $5.99/mo T-Zones plan. I unlocked my iPhone using PwnageTool for Mac, and activated the data using the "Big Boss T-Zones Hack" available via Cydia.
Apparently T-Mobile changed the IP addresses on their T-Zones servers about a month ago, knocking out service to many a subscriber like me who doesn't use a T-Mobile branded phone. An update to the Big Boss hack fixed the problem temporarily, but then my service went out again, this time accompanied by a new error message: "You are not subscribed to a cellular data service." I upgraded my iPhone to the unlocked version of the new 2.1 firmware, and reinstalled the T-Zones hack. No luck.
I lived without EDGE data for a week or so and then, towards the end of last week, set about finding a solution to the problem. Turns out T-Mo did some more reconfiguring of their data servers, and a new fix is required. This one's pretty simple. I set my APN (found in Settings > General > Network > Cellular Data Network on the phone) to epc.tmobile.com. And now everything's peachy-keen once again. Just thought I'd share.