My house server was remotely rooted, which really annoyed me. Determined to not let it happen again, I buried myself in Linux security for several months, reading all I could, and participating in comp.os.linux.security. Kernel 2.4 is going to open up a whole new kettle of fish, as will the recent release of dsniff. Many of the firewall configuration statements will need to be updated to include syntax for the new iptables. Things are changing so fast, it will be enough of a challenge just to keep the FAQ up to date.
Solid backups are quintessential to the security of a system.
This is painfully overlooked these days, with the focus so much on the network component of security. Also to be covered is encryption, such as PGP, GPG , or filesystem encryption . Also covered will be logfile monitoring tools and methods. I have been bitten by both. I also feel there is an overemphasis on the minutiae of computer security by many Linux Administrators, without understanding security in a holistic sense.