Well, the errant GUI behaviour seems to be fixed, but I'll never know
what went wrong during the 6.0-to-6.1 upgrade of SL.
After a bit of user arm-twisting, I did a yum upgrade of the VM host to
SL-6.2 (which worked perfectly!), and the Virtual Machine Manager issue
went away.
It'll remain an unsolved mystery, I guess.
My thanks to the SL team for another fine release.
Cheers,
Chuck