(1) Problem seems to be random in that I've had a few logins - maybe 5-10% - where flashing did not occur.
(2) kdesktop_lock seems to run as a pair of processes per screen. Flashing seems to occur when the parent of these dies while the child remains.
(3) I don't yet know why the parents are dying but once they do and another kdesktop_lock parent/child pair is created we thereafter have multiple child processes competing for the same screen.
(4) Possible cause: http://bugs.pearsoncomputing.net/show_bug.cgi?id=3025
--Mike