apt key sig error

he last half of the large number above is the key number, for example 437D05B5. You can check if the key is in your key ring with (as root):
apt-key list

If the key is there I don't know what is up, But if it isn't then you need to get the key and add it to your keyring.

As root:
gpg --keyserver hkp://subkeys.pgp.net --recv-key 437D05B5
gpg --export --armor 437D05B5 | sudo apt-key add -

http://ubuntuforums.org/showthread.php?t=345930