From Yahoo News, Tuesday:

"Background checks jumped more than 41 percent since Friday's shooting that left 12 dead and 58 injured during a midnight screening of 'The Dark Knight Rises' at an Aurora movie theater. Over the weekend, the Colorado Bureau of Investigation approved background checks for 2,887 people who wanted to purchase a firearm, the [Denver] Post said, an increase of 43 percent over the previous weekend."

So the absolute number is comparatively small when compared with the number of guns in circulation, but it's still up by more than 1,000.

And those seem to be just the "approved" checks.