Would like to utilize both the GTX285 (to take advantage of CS5's MPE in PPro) and the ATI4870 in a 2009 MacPro. I've read that it's been done by putting a second power supply in the second optical drive bay to power the GTX285 but have been unable to get the details on what PSU was used, how it was mounted, connected to the card, etc...

So my question is, has anybody here done that...and if so, would you provide specifics on how to do it and what exact PSU to utilize?

I have a GTX 285 in a dual boot / dual OS machine that I just built. I would think it would be a tight fit stuffing in a second power supply in a MAC case as you have suggested. From what I understand, the GTX 285 is quite a power-hog, so you would have to find out what the minimum power supply requirements are for the card, and then try to locate a proper-sized (both in terms wattage and physical size) power supply. I suppose you could try to mount the second power supply outside the case and jury-rig some sort of power cable to the card. Good luck.