I finally purchased a second monitor and fired up Aperture in dual-monitor mode. It's too bad dual-monitor support is broken.
8,192,000 pixels of dual suck
My main monitor is a Dell 3008WFP 30" LCD connected to an ATI Radeon X1900 XT in PCIe Slot-1 (x16 lane width). My second monitor is a Dell 3007FP 30" LCD connected to an NVIDIA GeForce 7300 GT in PCi Slot-4 (x8 lane width).
Using Aperture in any secondary display mode resulted in the primary display exhibiting two rendering anomalies:
1. extreme posterizing in 100% (zoom) view 2. some images render as black (no image) in the "normal" fit-to-window view
The secondary display content worked fine. Screenshots of the window produced no artifacts, but the rendered display looked terrible. The crap render persisted when I dragged the main Aperture window from the primary display to the secondary display, so it isn't a monitor or video card issue.
In frustration, I disconnected the second monitor from the NVIDIA card and re-connected it to the second DVI-D port on the ATI Radeon X1900 XT. Everyone seems to work now.
I kept the stupid NVIDIA card just so I could use it with a second monitor. Now, it appears better to have a single card than to attempt to use two.
Come on, Apple! Get your shit together. You only have to deal with, what, 4 video cards drivers on the Mac Pro? You'd think they would work flawlessly.