Hi Neil,
sorry for posting this late, was busy.
Yeah, I had the very same problem with a new machine here. eCS 20rc6 (note: not "-a") was unable to shut down, regardless what application (styler, x/e-center) was used. I reported it to Eugene who turned this into a Netlabs ticket. rc6a also failed with out-of-the-box driver levels.They pointed me to a new version of [ACPI-something] I had to download which fixed these problems wonderfully.
See ticket for further reference:
http://trac.netlabs.org/acpi/ticket/373HTH
Thomas