Note: This code has been written with WordPress 2.8 in mind. It won't work on versions prior to 2.6 and might have unpredictable results in later versions.
Drag and drop is a useful mechanism to do stuff and can turn a complicated set of buttons or drop down lists into a simple elegant solution.
Wordpress has lots of scripts built in, which do a lot of useful functions. However, they are not enabled by default, which means that in order to get your script to work you need to enable them.
The drag and drop functionality in Wordpress is provided by the Scriptaculous framework. To get Wordpress to include this in your page header just use the function wp_enqueue_script() with the parameter "scriptaculous-dragdrop".