I'm running WP 3.0.1 on CentOS with Shibboleth SP 2.3.1. The SP is configured correctly -- I can protect locations in Apache. However, with the shibboleth plugin, logging in to WP via Shibboleth causes the login redirect loop that others have noticed.
I have verified that "Shib-Session-ID" is present when authenticated via Shibboleth, so the potential problem that tsikorski noted does not seem to be the issue.
Any advice would be much appreciated.