the trick is to buy a feature phone (I prefer the RAZR). Sign up for your T-Mobile plan with that phone, add $10 internet, add whatever you want. Then, after your plan is initiated and the phone (RAZR remember) shows up at tmobile.com on your plan, pull the SIM and pop it in your BH2 (or WHATEVER PHONE YOU WANT). I don't even think you need to make sure it's unbranded, but I wouldn't know because all of my phones have been rooted with custom ROMs anyway.
Anyway. Change your APN to internet2.voicestream.com and away you go. You'll then come back whining about how your MMS doesn't work, but that's a different thread (hint: search my user name and internet2 and MMS).
I don't want to say this too many times because I rue the day that TMoUS pulls the plug and takes away our $10 party.
Your phone type will change to the new phone (BH2, HD2, etc.) on Tmobile.com after a few days/weeks/months but your internet should still work fine. Before I call Tmobile about anything, I switch my phone on mytmobile.com back to the RAZR and put the SIM in that phone before I call.
I know it's an old thread, but it needs good information.