Although you can easily change your "long" name in OS X, changing your short username is a messy procedure -- and that's assuming you're following the correct instructions -- even Apple's are incomplete. James Bucanek and I finally got fed up with this restriction and created ChangeShortName, a Unix script and helper application that allow you to easily change the short username. You simply provide the existing name and the new name and ChangeShortName does the rest. It even backs up your NetInfo databases for safety.
ChangeShortName is free, but please make sure you read the included documentation before using it!!
[robg adds: An older hint (and the associated comments) explain how to do this the tough way, along with a link to another name-changing front-end. I've never used any of these techniques, but this is a very popular request, so I figured one more tool couldn't hurt.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040527135936888