With GIMP 2.8.4 (from ftp://ftp.gimp.org/pub/gimp/v2.8/osx/gimp-2.8.4-nopython-dmg-1.dmg), running on 10.7.5 or 10.8.3, pasting graphics copied in other Mac apps (eg. in Preview.app) can not be pasted into a GIMP window.
It appears that GIMP keeps its own, separate, clipboard instead. Note, however, that copying graphics in GIMP do get synchronized to the shared system clipboard. Only the other way doesn't work.
In older GIMP versions, it appears that the fix was to change X11's preferences:
1) Click X11 > Preferences
2) Click the Pasteboard tab
3) Clear the checkbox next to "Update Pasteboard when CLIPBOARD changes"
However, this option isn't available any more since GIMP now works without X11, hence there's no X11 Prefs app any more. Well, in fact, I do still have a X11 app on my 10.7.5 system, but there this option is enabled, and it doesn't seem to help.