I tried to do an apt-get update on a MD and got a no public key - NO_PUBKEY XXXXXXXXXXXX error.
Where XXXXXXX is the number returned.
To recover I had to do the following 2 commands...
gpg --keyserver subkeys.pgp.net --recv XXXXXXXXX (the number)
gpg --export --armor XXXXXXXXX | sudo apt-key add -
After executing these commands my apt-get update ran fine.
apt-get dist-upgrade did not cause any problems, just apt-get update.