The real thing to note about the recent NBC/WSJ/Marist polls of Ohio, Florida, and Virginia isn't that Obama has a lead — it's the number of undecided.
In Virginia, Obama leads 49 to 44, but only 5 are undecided. Furthermore, only 2% say they might switch before election day. That means even if Romney swept all the undecided vote, he'd still only have a tie. He's got a structural problem in Virginia.
In Florida, same story: Obama leads 49 to 44, but only 5 are undecided. He could win all of them and still be no closer to victory. Same thing, only 2% say they might flip before election day. He's got a structural problem in Florida.
In Ohio, the news is devastating: Obama leads 50 to 44, and once again only 5 are undecided. If Romney swept all the remaining undecideds, he's still going to lose Ohio. Worse, only 2% might flip.
Obama isn't enjoying a "sugar high" bounce from the convention; this is a structural lead.
For Romney to win, he would have to do more than grab ALL the undecideds. He would ALSO have to convince Obama voters to switch to Romney. There's no way he can do that, unless he gets outside help (some external event, or Obama shoots himself in the foot).