The current RCP averages show Barack Obama's average national opinion poll lead over Mitt Romney to be 3 points better than his net approval rating (+3.3 versus +0.3).
I think that this +3 number may be fairly stable for Obama net approval ratings close to 0 (as they have been for most of the last few months), and that it's a fairly encouraging number for Obama when compared with Bush-Kerry. It's certainly one that's worth following as the election campaign progresses.
A good way of cross-checking this apparent positive gap between Obama's lead over Romney and his approval rating might be to look at state polling data where there is a series of data drawn by the same pollster, so using the same methodology. NC as PPP's home base gets polled by them every month, so I checked Obama's approval ratings against his head-to-head with Romney over the past 6 months. It was as follows:
April: Net approval +1, O-R +5 = Gap +4
March: Net approval 0, O-R +3 = Gap +3
Feb: Net approval -1, O-R +1 = Gap +2
Jan: Net approval -2, O-R +1 = Gap +3
Dec: Net approval -3, O-R 0 = Gap +3
Nov: Net approval -5, O-R -1 = Gap +4
So at least in NC there seems to have been a very stable relationship between Obama's improving net favorability in the past 6 months and his improving position against Romney in the polls, with the gap again averaging at +3, like the current position nationally.
Comparisons with Bush-Kerry do not flatter Romney. Generally speaking, Kerry was running a point or two better in the polls than Bush's net approval for most of the period leading up to the election. Only in a relatively brief period in May-June did Bush outperform his approval rating in the head-to-head by a few points; before and after that Kerry was level with or a bit better than Bush's net approval.
Although we're only talking about a few points either way, this could end up being pretty important given that Obama's net approval has been around 0 for most of this year. Current indications are that maintaining that level might well be enough, and that he might even win with a negative net approval of a few points. The magic approval number for incumbents is supposed to be 50, but for Obama it is probably nearer to 45.