There are multiple components of SELinux that a developer may be interested in.
Mail lists and IRC
Reference policy development is primarily done on the Tresys SELinux Reference Policy development mailing list (official archives)
SELinux developers can also be found on IRC at irc.freenode.org channel #selinux
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.
XXX Add todo page XXX
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
There are also several SELinux related projects that you can help on
Labeled NFS Adding MAC support to NFSv4.
XACE Adding Flask/TE support to X.org.
sVirt Integrating SELinux and Linux-based virtualization.
Apache/SELinux plus extending SELinux labeling over the web.