The problem ranges in severity to blacking-out the 2nd display, to blacking out both displays and requiring pulling the power plug (hard power cycle). It is an issue with the video card in the 2010 Mac Pro; the good 'ol GT120 card in the 2009 Mac Pro worked properly for me.
I am pleased to report that a new cable sent to me by NEC largely solves the Mini DisplayPort wake-from-sleep issue. Not perfect— but I think it’s fair to say that the remaining sporadic issues seen once every few days are fully accounted for by Apple bugs. I know that NEC is relaying data to Apple, so there is a reasonable hope that Apple will get its act together on this issue which has gone on for 7 months now.
The bad news is that the cable is not yet readily available, but when I have info on where to buy the magic cable that works, you’ll read it here asap.