How can I keep track of twitter usernames in Address Book? I want to add twitter names to my contacts in Address Book. But I cannot find an obvious place to do so.

The best fit would be as instant messaging username, but twitter is not listed among the services. I there a way to add a service here? Does anyone know where they are stored?

I could add a new "twitter" label, but I can't stand the "AIM" or "Yahoo" behind it.

Any ideas are welcome.
Thanks to the comments on Ipacheco's answer, I ended using a new url labeled "twitter" with a format of "tweetie:@someone" instead of the http:// link to the twitter homepage. This opens up tweetie for Mac and iPhone (now named twitter) whenever you click the link.