T-Mobile charges $20/month for unlimited BIS add-on to a regular minutes plan. It's the best deal in Blackberry data anywhere. You still need a text plan but you can talk them into unlimited texts for $10/month or less.
The thing to remember with Blackberrys is that you need the dedicated Blackberry Data Plan
, not a generic smart phone unlimited mobile internet/webmail plan. They include pretty much the same things, but one is specifically for Blackberry devices. The generic plan won't give you all the services you want on a Blackberry. Unlimited BIS is what you want