Some websites already have the functionality to shorten their URLs. For example on snaplog.com a URL can be accessed 2 ways. Content html pages would programatically describe the shortened URL by embeding a attribute in the page.
<link rel="short_url" href="http://snaplog.com/:E7d" />
Interesting idea. Rather than forcing users to use external shortening services, websites should provide their own short URL’s and provide a way for services like Twitter to find the proper short URL for a page.