There is so much speculation out there. I would not even exclude that a lot of hardware will work that isn't officially recognized by Apple. Officially the 3870 does not exist for Apple. It works absolutely flawlessly in every situation that I have encountered so far. I would not be surprised if it would work with Open CL as well. I will only exclude it if I have installed SL and recognize it will not work. Same goes with the 2006 and 2007 32 EFI Mac Pros. We will probably only know what works and what not when we have fitted the OS and run a trial.
But I can allready tell you that I'm seriously pissed off by Apple's attitude of treating customers with 2-5 year old computers. It is an embarassment that they even consider to not to issue firmware upgrades to those customers. They are exposing their most recent customers to massive devaluation because they cannot run the most modern OS features on hardware that is perfectly capable of doing it. It makes me very angry to be treated in that way.