That's true. I just threw this together. I think you could even create a database table and generate the redirect on the server side or dynamically create the javascript. Most browsers have javascript enabled. Maybe a hacker with some time (maybe I'll create one) could create a site with a dynamically created .js file and a remote link like this:

   <script src="http://takebackthewebfromurlshorteners.com/bannedreferrerslist.js">
  </script>

This .js could be dynamically generated from a database table of known url shorteners. Such a service could even get really fancy and show stats from all the known shorteners, etc.

You could add parameters like this:

  <script src=".../bannedlist.aspx?allow=digg.com,bit.ly,etc..."></script>

Then the user could customize exactly which url shorteners they want to allow.

All kinds of things a service like this could do!