Renaming Home Directory
From Mac Guides
This guide addresses changing the name of your home directory (the account short name) in Mac OS X.
Recovering From An Accidental Home Directory Name Change Through Finder
A common pitfall for Mac OS X users is changing the name of their home directory by simply altering the name in Finder the same way other files and folders are renamed. Unfortunately, this generally results in an apparent loss of the user's data upon the next login. This can be unnerving, but fortunately the solution to this problem is straightforward. Apple has posted this support document, entitled Return to default desktop, apparent "loss" of home directory, to assist users experiencing this problem.
Intentionally Changing The Name of A Home Directory
Warning: Apple recommends that users not attempt this procedure unless "absolutely necessary."
Apple has posted a support document entitled How to change user short name or home directory name, a forty-step guide to changing the name of a home directory in the safest way possible.
If you want to change your short user name another approach is to create a new account, and copy the folders inside your old home directory to a new folder on Macintosh HD, and then logging in as the new account and copying them into the new user account.
Alternatively Dan Frakes has a utility, ChangeShortName: Use at your own risk, read and follow all instructions to the letter.