We've been using the Radeon 5770 on our 2008 Mac Pro and works fine. There's also the users review in the Apple Store for Graphic Cards that says the 5770 works in older Mac Pros. I don't understand why Apple would release notes about this card not working with older Mac Pros.
Simple. They want you to buy a new Mac Pro instead. 5770/5870s work in all Mac Pros, but Apple won't provide actual product support unless it's installed in a "supported" system.