I put the Mac Pro to sleep around midnight, and woke it up around 7am to a headache-inducing display.
The problem is not unlike the problems I experienced with an ATI X1900 on the G5 Quad, as described in my Jan 31, 2007 entry. The problem is provoked in exactly the same way: everything is fine after a short system sleep, but things go haywire after sleeping overnight. As this has never happened with other brands of video cards I’ve used, it seems reasonable to conclude that it’s a problem with the ATI X1900, possibly induced by having two monitors attached.
What to do now? I’m keeping the Mac Pro. So perhaps the X1900 has to go.