(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