I've been using Ubuntu 6.10 for a while and thought it was about time to upgrade to 7.04.
I opened the terminal and typed.... gksu "update-manage -c". This brought up the update manager dialog box with the button to upgrdae to the nextr version of Ubuntu. I clicked the button and the upgrdae promptly failed. In the terminal window there was a message regarding "GnuPG exited non-zero, with code 131072".
This basically means that Ubuntu is trying to ensure that the downloads it wants to grab are really the right ones - it does this by checking the PGP keys against a signature - and cannot find any PGP setup on the local machine.
The fix is to run "pgp" in the terminal. Once PGP states "Go ahead and type your message" press Ctrl-C to break out and re-run the update manager with - gksu "update-manage -c".
Click the button and all should be well (as long as you've made a backup of course)