Case closed, (maybe)
Software updates are checked to happen automatically.
A week or more ago, when I was wrestling with a wireless network security problem, I reinstalled OSX from the disk that came with the iMac.
Today, I did a software update just in case updates weren't happening as they are supposed to do. Maybe I got lucky. Eventually, I got the OS X 10.6.8 V1.1 update...with 11 changes in all. After that, Keynote worked.
I don't know, but surmise reinstalling from the OS disk may have put something old on the iMac that didn't like Keynote. All good now, maybe. I will let you know otherwise.