I FOUND A FIX FOR THIS!
ZuneBoards guys, you may want to add this to the list of ways to fix the Zune rollback error for installation/upgrade!
After fighting and fighting with this thing, I did one desperate websearch, and came across this guy's blog:
http://www.lockergnome.com/digged/20...ort-is-a-joke/
One of the comments is from someone named "Justin", to whom I now owe a great debt. He somehow figured out that you can fix the rollback error problem by turning ON the Windows Firewall. That's right, turn it ON. I don't know how this has any effect, whatsoever, on a Zune's software installation, but I now have a happily Syncted and loaded Zune, ready for my flight to Vegas!