T-Mobile BlackBerrys do not roam to Cingular unless there is no T-Mobile network in the area, like in the US Virgin Islands. It will not roam to Cingular in a weak area.
T-Mobile has international roaming agreements with multiple carriers. Those roaming agreements do not extend domestically.
Similarly, if a Cingular phone was in a weak area for Cingular, it would not roam to T-Mobile USA.