I'm having the same problem as this, but the answer from there doesn't work for me.
Neither in my iMac upgraded from Lion to Mountain Lion (currently at 10.8), nor from a Mac Mini with Lion Server (10.7.4).
I have "Show fast user switching menu as" enabled at "Users & Groups" Login Options, but when I have to login from screensaver, there's no option to switch user.
How can I enable such an option?
EDIT: I think I forgot to mention that we believe (just guessing) that it has to do with network accounts. We use network accounts here, and have that issue. My boss says he could solve this issue at his house's Mac, which don't use network accounts. Does it make a difference?