Mark,
What I mean about the thrust being matched is that the thrust-to-weight ratio should be the same. That means the orbiter acceleration and the ET/SRB acceleration would match even if the vehicles were seperated. That way stress on the attachments would be minimal. The weight of the ET/SRB's is considerably more than the weight of the orbiter. That would mean that as solid and liquid fuel are burned, and the ET/SRB's become lighter, that the orbiter engines would have to slowly throttle up to avoid unwanted pitching, since the SRB's thrust remains constant, or nearly so.
Of course, there is the idea of a metal shield that would cover the underside of the wing, and jettison at the same time the ET jettisons. I'm sure they have thought of that.