The Princeton Election Consortium finds Mitt Romney actually got a negative bounce coming out of his convention.
"Indeed, it appears that the race shifted towards President Obama by 6-15 EV, or about 1.0% of Popular Vote Meta-Margin. From an analytical perspective, a negative bounce is quite remarkable because all the talk in recent weeks has been of bounces being smaller or zero, but always in the hosting party's favor. It is all the more remarkable because of the relatively small number of state polls over the last week, so that the Meta-analysis's inputs have not fully turned over… So the negative bounce may be larger than what is shown in the graph. Such an event would have been missed in past years (and even this year) because national polls don't have the best resolution."