Unfortunately I have an occasional problem where aptitude decides that my GPG key is bad and I get errors like this on my local proxy;
W: A error occurred during the signature verification. The repository is not updated and the previous index files will be used.GPG error: http://apt-proxy hardy-backports Release: The following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key
Now, I'm not sure whether the problem is with my local proxy or the ISP's mirror but to fix it I need to issue the following command;
sudo apt-get update -o Acquire::BrokenProxy=true
Everything should be OK from there. If you still have problems, I have also sometimes found that if 'sudo apt-get update' fails, using 'sudo aptitude update' instead works.
I have no idea why.