I thought URL shortening worked like tinyurl where you don't necessarly have a tag, and the urls are like:
TinyURL.com - shorten that long URL into a Tiny URL
Gilby's Website
Unicyclist Community
www.tinyurl.com/003
or some sort of number/letter extension. If your site got popular it might be hard to find unused tags. I tried the tag "test" and it was already taken. Not surprising as it was probably used to test the script. Maybe you would want to have the ability to auto-generate a tag.