Interestingly enough, that actually does work to a degree. I had tried that procedure before, but for some stupid reason I used the hal from SP6 and not SP4 and it makes a difference.
Now with the SP4 hal.dll, the VM automatically powers off, but here's the kicker - it only works when you do a shutdown from the Start Menu (inside the VM).
When you do a shutdown from the VM menu (VM -> Power -> Shut Down Guest), it behaves as it did before.
Similarly if you do a soft shutdown from vmrun, it still sits there with the 'It's now safe to shutdown' message.
I also tried calling shutdown from a shutdown script and configuring that script as the "Shut Down Guest Operating System" but the result was the same - the VM doesn't power off. I guess the registry setting and associated hal.dll mentioned in the blog only work when Shutdown is initiated via the start menu.
Any other shutdowns don't appear to power off the guest.