No surprise that Apple giveth and taketh away with apparent haphazardness.
There's a
Mac OS X Hints comment claiming booting a Quad G5 from a GPT drive volume.
Ahh, I'd overlooked Boot Camp support being an obvious example of necessitated GPT usage.
I noticed comments about APM vs. GPT volume size limits in Rentzsch's
Intel-based Mac Boot Incompatibility article (which may be out-of-date in spots).