Possible start - Cluster mirrors are exhibiting a regression in the cluster 4.6 release when running 3 or more legs in a mirror. The real problems are due to issues with the underlying lvm2 and device mapper changes introduced in the rhel 4.6 releases. It is expected that this will be addressed in the first errata kernel/utilities released. The recommendation is to not update your systems to RHEL 4.6 and Cluster 4.6 until those problems have been addressed.
The subject line has now been generalized to include notes for both RHCS and GFS
It appears that this content is included in the release notes here: http://www.redhat.com/docs/manuals/csgfs/release-notes/CS_4-RHEL4U6-relnotes.html