I have a card for my gaming box and Mac. A 6950 and "official" 5770, respectively.
The idea is to have only one machine. This "all-in-one" machine had a number of drawbacks when I was weighing my options, not long ago. So I went the simpler route, and ended up with two computers.
Now I'm considering it again, for various reasons.
So what I'm asking was, is a 5870 with custom firmware on a Mac any different than the same card on a PC?
From what I understand, yes there are some differences. A flashed card in a Mac Pro can used only one DVI port, so if you want to use a second monitor there can be issues, I could be wrong on this, but I believe it is the case. Some cards have issues on boot up where there is a grey screen for a few seconds before it boots into OS X, again not sure if this has been resolved.
I considered a flashed card but then decided I didn't want to risk having issues down the road, and paid the extra money for a proper Apple video card. You may never have issues down the road, but you are essentially fooling the Mac by using a flashed card, so I think its hard to say there may not be issues with future updates or Lion.