man i screwed this up once myself - i changed my short name manually somehow and i couldn't log on as my normal user anymore, all my preferences and files were gone, it was kinda scary.
i logged in as a root user and changed my name back to what it was manually again (i think in the get info box) and logged off, restarted and everything was cool again, but my advice is:
DO IT HOW APPLE RECOMMENDS or you might have to do some freaky stuff to get it right again.
i'm a total newbie, so i was schocked that i got it fixed myself, but be careful