I can't take credit for this one -- saw it on a thread on the *other* BB site... But for all us 8300 users on T-Mobile, it looks like good news.
If you go to Host Routing Tables and reregister - TMo will shoot you a new service book that for configuring your BB web browser. I see at least one new item in my favorites, and T-Zones now recognizes my phone as an 8300.
At least a couple of people on the other forum have reportedly succeeded in getting the standard TMo service books and browser with the latest 8300 OS (Voda I believe). I haven't had time to try this yet - either later tonight for first thing in the AM. But I think the 8300 is finally properly supported on TMo!!!