Comment 33 for bug 429841

This also affects me while upgrading from 8.04.4 LTS to latest 10.04 LTS. I've downloaded the ISO and started a cdrom upgrade.
Also worked for me after doing the fix suggested above by Emanuel Greisen:

rm /var/lib/dpkg/info/flashplugin-nonfree.prerm
dpkg --remove --force-remove-reinstreq flashplugin-nonfree
dpkg --purge --force-remove-reinstreq flashplugin-nonfree

all commands with "sudo" in front.

Hope this helps,
Fabi