As a former Crackberry owner (actually the handset is on a closet shelf), let me take a stab. TMobile and Cingul,ar Blackberries ship locked. Both will unlock them with the right reason, but they will ask. International travel and a prepaid SIM is fair. You'll need a history with the company, good bill pay history, etc. Lastly, with TMobile the phone cannot have been released within the last 90 days.
With an unlocked Blackberry, you'll be able to make and receive calls using the unit as a phone. Even with a data enabled SIM, you will not be able to browse or get e-mail. A few people using Blackberry Enterprise Server accounts have managed this, but most carriers lock down the necessary ports. Stories of people pulling this off are sort of like the stories we've heard about 900mz GSM installations in various places in North America. There was a little bit of truth and a great deal of wishful thinking.
International MVNO's such as a Yackie, Riiing, or Geodessa could probably open the necesary ports for BES users to get through, but if you using BIS (Blackberry Individual Server) with a different provider, it ain't going to happen. Similarly, you will not be able to browse the web with a data enabled SIM. Everything is hardwired to Blackberry settings.
In addition to Howard's Forum, you should also check out
http://www.blackberryforums.com/.
Stu