Essentially just go to http://www.facebook.com/username/ and click "Set a username for your pages" where you will be prompted with options to set up a custom URL for any of your pages.
It does not seem this is possible, you reach a "road block" when going to the url as a Facebook Page (instead of a user), with the following message,
You are using Facebook as [Name of Facebook Page]
To access this page, you'll need to
switch from using Facebook as your
page to using Facebook as yourself.
Best Answer
Other than your canvas url
No I do not think so. The only thing shorter I can think of is fb.me