Actually, t-mo has had it mentioned on their site for awhile now, search for "5" in the support area. (yes, a number is what they are calling the feature, couldn't they have thought of anything more catchy?). It is US CALLS ONLY, and currently not compatible with sidekick, blackberry or handheld devices and only available on a " '5' Capable phone". Nor is it compatible on any "FamilyPlan".
no clue as to why they make this limit, but to quote the FAQ:
Can Blackberry