FC4 KDE. A couple of times, after unlocking my screensaver, all windows lack frames, including new ones, and the old ones cannot be closed, either via the control bar right-click, or by File->Quit. Neither can they be maximized or minimized with right-click. This has only happened a few times so far, and fairly recently. I ran updates yesterday (April 20) but it happened again today. Log out and in and I am back to normal.
Switching user and back again, which fixes my other screensaver problem of the whole screen being slightly too big, does not help with this.

Your description almost describes what you will see if you start x windows manually without a windows manager.
Logging out quits the windows manager and logging in again restarts it.

Is this just a particular screensaver? If so, you may want to uninstall it.

I use the blank screen screensaver.

Many of the screensaver programs are a part of the Xorg distribution. The xorg.0.log might contain information on what happened. Also check the regular log. IMHO, finding the cause of the problem is the first step in finding a solution.

The frameless thing has not happened for along time, but the oversize screen has, both after unlocking the screensaver and when logging in. However, since installing a new LCD monitor, it has not happened again. The monitor (Neovo) seems to be self adjusting, so maybe I just don't see it anymore.