Be Very, Very careful. The new BL 3GS can not be upgraded to iOS5 with BB 06.15
T-Mob in the USA have, as you say, been testing 2100Mhz in very small areas but thay have been doing this for some time and I have heard of no plans to roll it out to wider areas and I believe that with their financial woes, they are unlikely to do so - although, maybe the money they received from AT&T will help...
Although T-Mobile are now using 1900 for HSPA+ see http://www.macrumors.com/2012/01/27/t-mobile-to-offer-additional-support-iphone-users-on-its-network/
