I just noticed that the lock screen window isn't the only one "buggy". It seems to happen with all windows that are opened while the particular VT is inactive (some kind of pop-ups for example, in my case - Skype Downloads window and various Gnome messages). The windows appear white and there is nothing where the title area and the borders should be (although, as it was with the lock screen, the window can be manipulated with the keyboard and mouse blindly).
As far as I understand, compiz uses hardware-accelerated textures and overlays to draw windows. Hence it seems that a texture allocation/initialization fails with NVIDIA on inactive VTs for some reason.
Based on this, I suggest a workaround for the bug. We must avoid window creation on inactive VT, so rather than just using Fast User Switch Applet one should lock the screen manually (ctrl+alt+L) and then use the Switch User button. Screensavers/time-based screen locking should be also turned off - use explicit lock instead. Fast User Switch Applet can also be configure not to lock the screen.