Usually, if you hold down command while dragging a file (or folder), the Finder will move it rather than copying. This works if you want to quickly move something between volumes (the default is to copy.) However, if you're dragging from a folder you can't write to, command has no effect.
Is there a way to make the Finder always try to move something if you hold down command, or an equally fast way use the Finder to move an item out of a read-only folder?