I’m currently upgrading my Ubuntu-in-VirtualBox at work from Jaunty to Karmic, but at first it didn’t work at all. Update Manager got stuck when I clicked the “Upgrade” button.
Due to the Big Corporate Firewall all Internet access must go through the the Enterprisey Proxy so I immediately suspected a firewall/proxy issue. However, Update Manager, apt, etc have worked fine for a long time, so it must be something upgrade-specific.
Some debugging and googling led me to the url_downloadable ignores proxy settings bug report. Maybe I should have enabled jaunty-proposed but to get going I simply applied the attached patch and was able to get started with the upgrade.