I have a first gen Mac Pro (1,1) and I bought this card before I even heard of Bitcoin. I have been using it for a while now in both OSX and Windows with no flashing required. I even overclocked the core, downclocked the mem and reduced the voltage a little using MSI afterburner.
As a mac guy I am not happy saying that but the card even works better in Windows for several reasons. The apple OpenCL is super slow, it's almost twice as fast in Windows or Linux. It's supposed to be better in Lion but I haven't tested it. Also, AFAIK, there is no way to monitor the GPU temp in OSX or to control the fan speed. This makes mining risky and the card will most likely run way too hot. In Windows, If I don't apply custom fan speeds the card gets close to 100C and that's no good at all obviously. I have no way of monitoring the card in OSX but I bet it runs almost as hot even with the poor OpenCL.
As an experiment, I added a unflashed PC 5830 to my Mac. I don't think those cards even have mac roms available anway. The PC Sapphire 5830 works well side by side with the Apple 5870 and I got no problem so far.
For the record, I OCed the 5870 to: 1150 mV, 900 MHz Core, 325 MHz memory. The 5830 runs at 966 core, 325 memory. The system is super stable and stays decently cool for mining with these settings but I need to up the 5070 voltage and memory to the default settings for gaming or else the card will crash.
I get about 400 MH/s on the 5870 and 300 MH/s on the 5830. While gaming, i use -f 240 with my 5870 miner and the card still gives me about 300 MH/s with no significant FPS loss which is not too shabby.