Hello,
This has been a longstanding bug I've experienced since 4.3, still here in KDE 4.4.4.
Images speak louder than words in this situation:
http://upload.gaiatools.com/files/badlockscreen_0.png
As you can see, the password field, the text around, the OK/Cancel buttons, etc. do not show up on the unlock screen. I can still type my password and press Enter to unlock the screen. I can also press Escape to close the dialog. This makes the bug tolerable, at least.
So, anything I can do about this?

I compositing enabled? Have you tried disabling it? Have you tried disabling widgets on screensaver? Does this problem occur under a new user account? Which widget style and window decoration are you using?

TheBlackCat wrote:I compositing enabled? Have you tried disabling it? Have you tried disabling widgets on screensaver? Does this problem occur under a new user account? Which widget style and window decoration are you using?

Compositing is enabled; disabling it gave the same result (except the background, which wasn't black anymore but the default KDE one).
Disabling widgets on the screensaver didn't change anything (besides removing the widgets obviously), with composition enabled and disabled.
I am using the "Blank screen" screensaver but all other screensavers do the same thing.
I am using the Oxygen widget style, Obsidian Coast color scheme, Oxygen icon theme, Oxygen window decorations, and KWin as window manager.

I tried creating a new user and... it worked! With the same widget style, same window manager, same screensaver, same color scheme, same window decorations.

So yes, it's probably something to do with my configuration, but I have no idea what.

I confirm the same problem on my Fedora 11 64 bits
I also did the same and tried on the guest account I have and it worked fine.
I can't switch to use another user account for now nor messing up with my current account. I will try to reproduce the same problem on the guest account.
if some one find out how to resolve the problem please post a message
in case you solve it WindPower please post a feedback and tell me how you did
thanks

I did not fix it yet sadly. It's probably something I installed along the way while setting everything up. I will be careful and watch for this next time I set things up, but it'll probably be a while until then.

One interesting observation: In KDM's configuration, if I check "Enable Auto-Login" for me but also check "Lock session" (so that my session pre-loads when my computer starts), then KDM disappears (obviously) and plasma is locked instantly, but no widgets show up (which is a good thing). And here's the good bit: the unlocking controls DO show up for this initial unlock. All further locks cause the unlocking controls to disappear.

Disabling auto-login and starting the session locked do not change anything to the problem.