Pat, "yum clean all" had no effect, however your reply pushed my
investigation into a new direction and I was able to figure out what was
wrong. Following an advice from this list, some time ago I had modified the
repo file for sl-fastbugs, by replacing "$releasever" with "6x". I believed
(wrongly) that this was all that was necessary to upgrade the system to 6.1.
In fact I should have replaced every occurrence of "$releasever" in all SL
repo files. Not having done that I ended with a 6.0/6.1 hybrid, which
explains the dependency problems. I have now modified all repo files and
with this change all dependencies resolve correctly. So I consider the
problem solved. Since the server works OK and is current with security
updates, the proper upgrade to 6.1, including the packages that I complained
about, will have to wait until next maintenance day.

Thanks a lot for your help.