ChangeShortName is an easy-to-use utility that does the dirty work involved in changing the short name of a user's account in Mac OS X (versions 10.4.x and 10.3.x). It can also repair some problems that occur when a user has attempted to change their short username manually and done so incorrectly. For safety, it backs up the NetInfo database beforehand.
Changing your short username is a serious procedure, but ChangeShortName makes doing so easier and safer.

ChangeShortName is "donationware" by James Bucanek and Dan Frakes. Be sure to read the included documentation before using it.