I've only tested this with GIMP 2.8.0 from http://gimp.lisanet.de/Website/Download.html
. There's a new build for Mountain Lion.
1. Remove all versions of X11 (shouldn't be there after upgrading to Mountain Lion) and XQuartz from all locations like Applications and Utilities.
2. Download and install the newest version from http://xquartz.macosforge.org
3. Reboot and start GIMP. When prompted to find XQuartz, look for it in Applications>Utilities. Dragging and dropping an image file starts a timer that won't let you finish finding XQuartz. After GIMP opens successfully, drag and drop should work.
And, yes, it rains in California. When it does, towing companies, auto body shops, chiropractors and personal injury attorneys rejoice.