Dan Walsh's Journal.
This link contains a lot of practical SELinux information about configuring things properly.
If you are interested in picking up a book to learn about SELinux there is one called SELinux
by Example. It is based on FC4 or 5 but most of the information is still relevant. I own this
and the O'Rielly book and I would recommend this one.
Earlier in the comments is a page called equivocation.org which seems to have a reasonable
amount of solid SELinux information.
In addition to this is www.selinuxproject.org which contains a wiki with information.
I am also preparing a series of tutorial materials for SELinux but they are still in the works
and I need to get them reviewed by work before releasing them. However once they are public
they will be on selinuxproject.org
If you are interested more in the foundations of SELinux and its access control model there is
an email that was posted to the IETF working group mailing list for NFSv4 that provides links
to papers on why MAC is important and which explain the underlying access model for SELinux.
The email can be found the link below.