MacPro 3,1
I installed an EVGA Titan on the 3,1 with an additional internal power supply inside the 2nd optical bay.
Not easy would be an understatement. I didn't want to stress the MacPro's PSU so I bought an additional power supply. Routing those cables through the existing chassis was quite a task.
Although the Titan in theory should run around circles around the GTX 680, I am not quite sure it has any additional advantages over that particular card in OSX applications. All I can tell you is that the current drivers provided by NVIDIA or 10.8.4 do support that card (although you are not going to get the boot screen). Has not been a really big deal to me as I noticed that normal booting sequence will hunt for the first valid boot up disk if the designated boot disk fails to start the MacPro (I would make sure you have more than one valid boot disk).
Why did I stick a Titan on the MacPro? I just wanted to see if I could.