Commit 2b25000bf5157c28d8591f03f0575248a8cbd900 ("Restore force switch of md
array to readonly at reboot time.") causes a reboot/shutdown to hang
indefinitely on my box. Reverting this single commit makes the problem go
away. It was first released with 2.6.27-rc3, I believe, and so this is a
regression vs 2.6.26 (Rafael CCed).
I think the problem might be because my rootfs is on a RAID5 and my distro
fails to stop it completely before halt/reboot.
Please let me know if there's any more information you need from me.