There is actually an app called
ChangeShortName, and it works great on Tiger - I've used it in 2 machines. It will work in 10.4 and earlier, but not 10.5.
Supposedly you can change the short name in Leopard right in the User Account in System Preferences, but after trying it a whole bunch of different ways, it doesn't work. I wind up with a fresh Mac and no access to my things until I change it back to exactly what it was.
Actually, in Leopard, I totally reinstalled the OS, and restored all my files from a Time Machine Backup. I used a new short name, and had no problems with anything.
Anyhow, yes, that app does just what it should in Tiger.