Just a quick post, because I always have to look it up: If you get a message like this while installing a new package through apt…
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 C514AF8E4BA401C3
… you can do the following to get rid of it:
gpg --keyserver pgpkeys.mit.edu --recv-key C514AF8E4BA401C3gpg -a --export C514AF8E4BA401C3 | apt-key add -
Works on Ubuntu and other derivatives of Debian all the same, of course. Hope this post will safe me some time the next time I run into this.