There are multiple components of SELinux that a developer may be interested in.
Reference policy development is primarily done on the Tresys SELinux Reference Policy development mailing list (official archives)
Users and vendors who are interested in developing SELinux policy should go to the Contributing to Reference Policy page at the Reference Policy project site.
To develop patches for the kernel security subsystem, use git to clone the security-testing-2.6 tree
Information on Adding New Permissions to the kernel.
If you are looking for something to do the Kernel TODO has many ideas to get started on.
To Contribute to the SELinux userspace see the contributing page on its project site.
Userspace object managers
If you are interested in developing userspace object managers for SELinux see
XXX likely will put a copy of http://docs.huihoo.com/selinux/gconf07.pdf in wiki format here, or something from sepostgres XXX