What is the best way to learn SELinux?

Fedora's SELinux documentation is a good place to start. While referring to Fedora 13, the SELinux User Guide has plenty of information about how SELinux works. I also recommend reading Dan Walsh's Blog, where he talks about SELinux and related issues. Lastly, drop into #fedora-selinux on the FreeNode IRC network, there are often people there who can provide some input.