A fellow user came across a problem, and it's the exact same one that I'm having. I'm just reposting here to see if there's a faster response (original Ubuntu Forums post):

So I have upgraded to oneiric and I was excited to get gnome 3 and so
i did and it's great, but in the applications menu many of the icons
are showing double, which is strange. One will be clear and the next
will be pixely. Also, this only happens when I'm in the "All"
category, and with the Faenza icon set the pixely one is not in the
Faenza style. I'm not quite sure how to fix this problem and I need
some help.

A Google search turns up mostly closed Fedora-related bugs, so I'm not sure what I should do to try to fix this. Any help would be greatly appreciated!