If you come across this error after running aptitude update or clicking reload button in Synaptic:
W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available:
W: You may want to run apt-get update to correct these problems
due to a missing GPG key do the following to correct the problem:
$ gpg --keyserver subkeys.pgp.net --recv 58403026387EE263
Replace the key number “58403026387EE263? with the one included in your error message:
$ gpg --export --armor 58403026387EE263 | sudo apt-key add -
After that, the key will be added to a list and the error will not reappear.