We've released version 0.09 of RT-Authen-ExternalAuth. This is primarily a bugfix release. It ensures compatibility with all of the changes made in 3.8.10 and 4.0.0. If you've been having difficulty deploying RT-Authen-ExternalAuth or have been delaying an upgrade, please give it a test.
The extension is available from CPAN: http://search.cpan.org/dist/RT-Authen-ExternalAuth/