This afternoon, Gallup applied its likely voter model for the first time and found Romney gaining 5 points compared to registered voters. But while a persistent likely-registered voter gap isn't good for the president's chances, it's worth remembering that this is about the same difference that Gallup found at this point four years ago. As you can see, Gallup initially showed a 6 point gap between likely and registered voters, which shrunk to just a couple of points by Election Day.
The y-axis is the gap between likely and registered voters.
This isn't necessarily good or bad news for any candidate. It just shows that the Gallup numbers don't point toward anything unusual or unprecedented.