If you ever got the GPG errors while updating your Ubuntu sources, here's what you have to do to overcome it:

Example of the error might be like this:

W: GPG error: http://ppa.launchpad.net lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EF4186FE247510BE

And you just need to execute these two commands:

# gpg --keyserver hkp://subkeys.pgp.net --recv-keys KEY
# gpg --export --armor KEY | sudo apt-key add -

Just replace ke KEY with the one that exist on the error. According to the example, the command should be like this:

# gpg --keyserver hkp://subkeys.pgp.net --recv-keys EF4186FE247510BE
gpg: requesting key 247510BE from hkp server subkeys.pgp.net
gpg: key 247510BE: public key "Launchpad PPA for Ubuntu Mozilla Daily Build Team" imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)

# gpg --export --armor EF4186FE247510BE | sudo apt-key add -
OK

Then you can continue with:

# apt-get update

That's it. Why don't you try it yourself now :)