Regardless of whether Obama's recent comments in San Francisco were accurate or not, this flap will, without a doubt, hurt him among the pro-gun bloc in the GE. I don't see this will have an effect big enough in PA so as to hurt him against Clinton, but McCain was just granted a gift.
Let's not remember that McCain is backed by the mainstream media, which is traditionally pro-Republican and detroys the Democratic candidate during the summer and fall.
Obama essentially said that he's not winning in PA because people without jobs cling to guns. The message he's sending voters is that Obama is an anti-gun candidate. This was a bad move, IMO, since as I said, the pro-gun vote is significant and necessary to reach the White House.