The Short Name is the name that Mac OS X assigns to your account and gives to your home folder, as seen in the Finder sidebar. Because of the how Mac OS X references your user account and information, the Short Name has never been something that could easily be renamed.
It still isn't as straightforward as you might think, but luckily Macworld's Dan Frakes takes us through the process and explores some alternatives that could prove to be just as useful.