Your best options at this point are:
1. Downgrade to Mac OS 10.5.5
2. Calibrate the monitor on another Mac or PC and then move it to your PPC machine (the calibration is valid, but you will need to manually copy over the ICC profile, and make sure that the monitor is still in "Programmable" mode).
I have just ordered a NEC LCD2690WUXI2-SV from B&H. It will be attached to a PPC Mac ... which has been upgraded to 10.5.6 (sigh!). It does not appear this problem is going to get fixed anytime soon -- or am I wrong about that??? Is it known that this problem applies to all PPC Macs with all graphic cards? Or might I get lucky?
Your option #1 above does not seem viable, at least in my situation, since Apple strongly recommends erasing the disk before downgrading.
As to Option 2, I see two possibilities. The mac has two internal disks, one with 10.5.6 and one with no OS. I could install 10.5.5 on the second disk and use that for calibration of the monitor. And then use the 10.5.6 for ordinary work. Is that a good solution?
What does your comment about being sure the monitor is still in "Programmable" mode mean (or will that be obvious when I receive and read the manual?)?
I also have an Intel based laptop. Would that be a better approach than using the second internal disk?
(The Mac is a Dual 2.7 GHz PPC G5 with an ATI Radeon X800 XT graphics card.)
Thanks for any clues.