notifications stop working

[Neal Becker]

This is on stock F11 kdebase-4.2.3-1.fc11.x86_64

After a while, notifications stop working.  When kmail receives mail, for 
example, I hear the sound notification, and the icon for notification shows in 
the tray, but the actual message plasma is not displayed.

.xession-errors says:
QCoreApplication::postEvent: Unexpected null receiver