There is a missing step in the Debian installation guide, you must import
GPG keys before you can add the new repo:

gpg --keyserver pgpkeys.mit.edu --recv-key  8B48AD6246925553
gpg -a --export 8B48AD6246925553 | sudo apt-key add -

and also the key 7638D0442B90D010

gpg --keyserver pgpkeys.mit.edu --recv-key 7638D0442B90D010
gpg -a --export 7638D0442B90D010 | sudo apt-key add -

More about PGP and APT:

http://ccm.net/faq/809-debian-apt-get-no-pubkey-gpg-error

Regards,
Adrian Mihalko