I think Roosevelt was just looking for the right excuse to come to Britain's aid anyway, it was just a matter of convincing congress and the American public at large. Like you said evan, the US was very isolationist up until then. Pearl Harbor provided that "excuse" in spades!
Perhaps Hitler would have eventually declared war on the US anyway, due to the unofficial aid America was giving Britain and the USSR in supplies and, to a point, convoy escorts.
Likely, again like you say, the US would undoubtedly have locked horns with Japan anyway, probably in the Philippines or Marshall Islands area.