I see that bit.ly have integrated with other URL shortening services to allow for custom domains in the short URL version.
E.g. When I shorten a URL from an article on TheNextWeb.com it gets shortened to tnw.co as the below illustrates:
gets shortened to:
Even if I do this through bit.ly it will return the above shortened URL.
How can I get bit.ly to use my own URL shortening service for pages on my site?