I encountered the same problem. After an apt-get upgrade, I got new versions (11.1) of the packages google-gadgets* and libggadget*. The next time I logged out of KDE and try to log in again, I got the "Plasma workspace closed unexpectedly" message, followed by an empty functionless black desktop.
After downgrading the google-gadgets* and libggadget* packages back to version 10.5, the problem went away.
