It makes sense why you need BIS -- over in the WiFi forum I've figured out that if your default browser is set to Internet Browser (you may not have this, I think it's a BIS Service Book) your WiFi traffic proxies through blackberry.net - other people have confirmed.

So - no Internet Browser (BIS Service Book) means your wifi is *not* proxying through blackberry.net, and instead you're doing direct IP browsing (i.e. a normal PC, etc.). Since UMA has to go through the T-Mobile/RIM servers in order for it to work technically, then your WiFi has to in fact proxy through those servers (at least that's the way I understand it to be implemented - you can't just use it through a random IP). But if you don't have the Service Book which adds that feature (enables, configures, whatever) then there's no way for your UMA traffic to make it onto the T-Mobile network.

Open Options -> Advanced -> Service Book. Do you see two entries "Wi-Fi TCP/IP Transport" and "Wi-Fi TCP/IP Browsing" on your device? I'm not 100% sure, but I think those are the books controlling this setup (since it's where the IP addresses of the gateways on blackberry.net are listed)