There is no reason for an unlocked 3G handset not to operate on a different 3G network for voice and text (some rare exceptions apply). For blackberry services now you may need to contact Telcel and ask if they offer a blackberry data plan.
I guess that is the "rare exception" the-economist referred to. Yes, T-Mobile uses different 3G radio frequencies. You will get EDGE on Telcel. Voice, text and data will work, but at EDGE speeds and not 3G.