SANS' Information Security Reading Room
— not specifically Linux, but tons of advanced reading here.
Lists hosted/followed by insecure.org
The whole shebang
— got an itch? Get it scratched here.
Full Disclosure archives
— rough, uncut, uncensored real-time security information.
— run by
— Bruce Schneier's monthly newsletter.
— the Common Vulnerabilities and Exposures list. A community effort to provide a common catalog (and name!) for security vulnerabilities.