Thank you, Consumerist. After posting my complaint, I quickly received a call from the executive office telling me that T-Mobile would credit my account the full amount of the spurious overage charge.
My one criticism is that the executive escalation representative tried hard to make me feel guilty about accepting the concession--if you're going to be paying anyways, why not do so graciously, and in a manner designed to elicit goodwill, rather than grudgingly at blogpoint?
No matter, the point is that T-Mobile responded quickly and decisively to keep me as a customer. Verizon and Cingular, take note!