I won't post a script but I can clear your mind about the process of writing it.
You have two options. Or you use a notifier, that will buzz the program when the directory changes, or you can run a daemon that will stay in the background and check the Desktop every x-minutes, which can be resource hungry (polling).
I'm not sure if there's a notify library on OSX, on Linux there is the inotify API implemented in the kernel. Also, the notification of changes system might already be implemented in the language you will use to write the script.
Then the rest should just be about checking if there's content on the desktop and moving it to the other directory.