Bob Hall and Democracy North Carolina have an excellent new analysis out today about county by county voting trends in the 2008 election that I'll delve into later, but one of the key facts in there is that 253,000 voters utilized same day registration.
Overall Obama won early voters 57-43, and since folks who used same day registration were 36% black it seems safe to say he won those ones by an even wider margin. But let's conservatively say he won the same day registrants by an identical 14 points to early voters in general. That would mean he picked up 35,000 votes on McCain with that group, a number much greater than Obama's overall 14,000 vote margin of victory in the state.
Now perhaps some of the folks who used same day registration would have registered during the standard period if that hadn't been an option- but I bet most of them would not have. One of the key moments for Obama's victory in the state may have come while he was still lagging Hillary in the polls during the summer of 2007, when the General Assembly passed same day registration.