This has happened every time I power up my Powerbook into OS 10.3.4.
Once the desktop loads, in place of my name in the Menu Bar, where I would go to switch users, is the word "Unknown". When I click on it, nothing happens, the menu won't even light up.
The only way I have been able to get my name back there is to go into Sys Prefs - Accounts, and turn off, then turn on Fast User Switching. Then everything is OK.
Anybody else have this issue? I don't have a lot of system hacks or anything. I do use Synergy which has Menu Bar buttons, but I am not sure if that has anything to do with it.