While upgrading from 11.10 to 12.04, I'm stalled at the step:

Installing new version of config file /etc/init/udevtrigger.conf

The Terminal in the Distribution Upgrade window is expanded, there is no prompt for user input. I've hit enter a few times but it doesn't change anything but add a blank line after the last print statement. It's been sitting for ~6 hours at this stage.

No other windows or dialog boxes are open. The upgrade has progressed through "Preparing to Upgrade", "Setting new software channels", and "Getting new packages". Under the progress bar (which estimates 1 day 2 hours remaining, and has for some time), it says Preparing to configure libpcap0.8. I'd take a screen-shot, but it won't let me save the file anywhere. The progress bar is about 1/3 complete.