I'm on Mac OS X 10.8 and I'm using VirtualBox with Ubuntu 12.4 as guest. After install "Guest additions" Drag'n'Drop was working.
But since I install "additional drivers" and reboot the system, it does't work anymore.
Do you have a solution ?
In Settings>General>Advanced I tried Drag'n'Drop "Bidirectionnal" and "Host to Guest"