According to this thread
on Apple's PMG5 forums, people who have purchased a late 2004 single 1.8GHz PMG5 are having problems shutting down when the display is an ADC Apple Cinema display..
The problem happens as thus:
You shutdown from the Finder... your apps quit, your account is logged off, your desktop fades to blue with the rotating gear, your hard disks and fans spin down, but both the display and the PMG5 are still on.
The only way to shut it off at this point is to press and hold the power button or unplug the power cord to the G5.
This has happened to many people who all had the same G5 model and an ADC cinema display in that thread, including myself.
The current hypothesis is, the late 2004 SP 1.8GHz PMG5 has only a 350W PSU. The draw of power from the ADC Cinema Display must be too much for the system to manage during the shutdown state.
The current solution I've found is to use Apple's ~$90 ADC-to-DVI adapter box to essentially power the Cinema Display so your PMG5's PSU can focus on other items.
I didn't find this issue mentioned on G5 Support, so I figured I'd stop lurking and contribute.
I hope this helps someone out there.