I was trying an update of the Ubuntu 12.04.2 LTS installation on my Lenovo T61. Everything was OK, but when the process reached the configuration of the nautilus-dropbox package, it stopped. I waited more than 15 minutes, but there was no continuation.
Google search pointed me to the Ubuntu forum and I’ve found a comment of Felix, who tried to upgrade to 12.04 and had the same problem. His last post contained the solution for my problem, too:
I switched to the text console and rebooted my notebook. After restarting and re-login, I started the package manager. It complained
dpkg was interrupted, you must manually run 'dpkg --configure -a' to correct the problem, so I opened a terminal window and entered the command
dpkg --configure -a. It stopped again at the nautilus-dropbox configuration, so I just interrupted this part with Ctrl-C. After that, the configuration continued till the end.
After rebooting, the package manager is alive and
lsb_release -a shows