There have been a number of new keys issued for PPA on the Launchpad site. If you are getting an error similar to the following, then here is how you fix it.
W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available:
NO_PUBKEY 60D11217247D1CFF
W: You may want to run apt-get update to correct these problems
Execute the following command replacing
gpg --keyserver subkeys.pgp.net --recv _missing key_
Followed by this command:
gpg --export --armor _missing key_ | sudo apt-key add -
This will download the missing public keys and add them to the apt key repo.
Note: There may be multiple keys that are missing. Simply repeat for each missing key.
UPDATE: ‘blackgr’ on the ubuntuforums released a script to automate this. If you have an account there and are missing a number of keys, this can speed things up considerably. This script can be found on post #43 on this page: http://ubuntuforums.org/showthread.php?t=1047743&page=5
UPDATE 2: Here are details about what is happening. http://news.launchpad.net/ppa/adding-a-ppas-key-to-ubuntu