For those that feel comfortable flashing their BIOS (note: may void warranty) and they are seeing DP issues on their HD 5870 could I ask that you try out the following BIOS:
http://techpowerup.com/vgabios/61848/ATI.HD5870.1024.091112.html
Wow. I'd be very concerned with possibly bricking my brand new 5870 (although I do have 2 - still) via BIOS flash.
Does ATI have a tool like NiBitor (which I've used to flash Nvidia cards)?
And, while I am incredibly appreciative of the effort, I do feel as though I'm being asked to beta (or even alpha, since we're talking BIOS here) test ATI's hardware for them. This Eyefinity feature is being touted very highly for something which is this buggy. I certainly wish ATI luck with it, and I'm willing to help in any way possible.
But I can't help but be a little disappointed. :(