Mystery solved, Urs.  Entirely my own fault.  There was an SL_LIVECD 
directory with gzipped password and shadow files in it on one of the 
hard disk partitions that must have been left over from playing around 
with an earlier live cd/dvd over a year ago.  I had completely forgotten 
about it, if I ever knew it was there in the first place.  When I 
removed that, booting proceeded with no problem.  Sorry to have wasted 
your time.

Stephen Isard