Larry Sabato collects the data on post convention bounces. They almost always happen, but they often don't always say much about November. In the past twelve elections, the Democratic nominee failed, like Obama has failed, to bounce three times. The most pertinent example is, I suspect, John Kerry who remained at 48% - a point or so over where Obama is now and about where Kerry wound up in the end.
My theory is that we are entrenched. If I am right, McCain won't move either. If Palin knocks one out, it may rub off on the ticket. If, ironically, McCain finds the post-partisan voice that Obama turned away from, he may move. But I don't expect the numbers to change.