NEC has posted a technical note on the Mac Pro video issues. The problem is due to cables not being made to correct specifications.
Cause — Certain Mini DisplayPort to DisplayPort cables available in the market were incorrectly manufactured and do not adhere to the DisplayPort design specification. This defect can cause the above symptoms to appear. The defective cables may be marked or sold as "DisplayPort certified" and contain the DisplayPort logo. These cables are not manufactured or sold by NEC.
I am investigating what solutions are available, and will be posting an update when I have concluded my research.
Today I tried the Monoprice.com “5714 Mini DisplayPort Male to DisplayPort Female Adapter” and “5987 6FT 28AWG DisplayPort Cable”. This combination, reported by one reader to work, hung my Mac Pro at boot with both screens black—unusable.