Here is a good article on what SELinux in RHEL 5 has brought to the table. Some cool new features and a lot more protection:

http://www.redhatmagazine.com/2007/05/04/whats-new-in-selinux-for-red-hat-enterprise-linux-5/

There is also some interesting points in the comment section. Basically, a reminder that NO security mechanism is 100% safe, but it’s better than nothing and should always be considered when looking at what you need for your deployment.