I've learned some more.
I started two upgrades, both amd64 11.04 to 11.10, using standard gui prompt.
One machine is a Dell OptiPlex 745, 2 core Pentium D, the other a Shuttle x27d, an Atom D510.
The installs both "crashed" at similar points: Some user feedback was required (the OptiPlex needed an answer to whether it should replace or leave alone some NetworkManager config file; the Atom needed an answer to similar question about apparmor config file) and the screen timed out. The screen saver came on (it's just to black, turning off the display) and then the machine appeared to be frozen. Wiggling mouse, hitting various keys and so on had no effect. Getting in with vnc and poking gui buttons and so on had no effect. Logging in with ssh, ps showed
eating up vast time.
I googled a bit and tried various Alt-F1 kinds of things on the OptiPlex to no effect. Googled some more and learned that I should probably have killed the gnome-screensaver before doing anything else.
I had not yet done anything to the Atom. I logged into it with ssh, killed the gnome-screensaver. The screen was still dead on the actual machine, but when I went in with vnc I was able to poke the button that it appeared to have been waiting for; the installation proceeded and now it seems to be alive with everything as expected in the upgrade.
Unfortunately, when I got around to trying to fix the OptiPlex and killed gnome-screensaver, whatever keystokes I had given it had queued up and caused all hell to break loose. I eventually got to the point where I saw no choice but to reboot.
It would not come up. It hung early, with no clue about where.
More googling led to a successful reboot on the previous (11.04) version of things. I then did the google-suggested
sudo apt-get update && sudo apt-get upgrade
and that led to some dpkg thing that ended up doing the complete upgrade successfully using command line.
So it's all better now.
Now I face the irritation and annoyance that the Powers That Be think my computers ought to look like iPhones rather than computers.
Thanks to anyone who thought about trying to help.