The way to solve the issue is to get the public key for the repo. This key is used to sign the repository so you know the repo is who it says it is. While apt-get will work without the key, it is there for a reason, and that reason is to protect you from unauthorized software being downloaded onto your system.

Sadly, I can't seem to find that key on the MIT pgp keyserver or on other key servers. I'll ask Niels Breet if he knows where one can get the key to stop these errors.