After June 11th, 2010, you might run into the following error while running
W: GPG error: http://packages.dotdeb.org stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E9C74FEEA2098A6E
That's because Dotdeb's repositories are now GPG-signed. Over at their blog, they've posted a quick, easy fix that gets you the new keyring package.
Paste the following lines into your terminal and you're done!
gpg --keyserver keys.gnupg.net --recv-key 89DF5277 gpg -a --export 89DF5277 | sudo apt-key add -