Update December 2013: I have a long blog post just on this topic: Improving a Rounded Upper Back
Recently I observed someone at the gym I've advised before on improving his head-forward, rounded upper back posture(thoracic hyper-kyphosis). My exercise advice was briefly to exercise the upper back and do much less exercise for the front of the torso but to stretch there instead.
What I saw was a commendable emphasis on working the posterior shoulders, but unfortunately always done with the spine rounded forward. The loads were actually pulling the spine more forward as the arms pulled back, for instance doing a row. And if I had to choose, for most people I'd say that improving the spinal curvature is more important than improving the shoulder position.
Generally exercise should be done "spine neutral," meaning the spine keeps its slight natural curves but isn't allowed to bend more. Indeed, the spine musculature's primary role is in preventing movement(which should be in the hips, shoulders, and limbs), although the thoracic spine does contribute to rotation(transverse plane), and maintaining mobility is good for all the joints. A rigid spine isn't the goal.
Taking It to the Park