All we appear to know for the Fermi-class cards is that the injectors work OK on a real Mac Pro with a 480 supported by a boot helper (in my case a Mac 285 in an 08 Pro) and the Quadro 4000 drivers installed.
The problem with the PC cards is that it needs someone to take a chance, buy one and let everybody else know. In my case I wanted CUDA support under bootcamp so spent the dosh, and luckily now we have the Q4000 software it is OK under Snow Leopard, including CUDA. My 480 has had to go to someone else and I am facing same issue as you over 580 purchase......
If anyone has injected the 460 please can they post here? The OP should note that we will probably always have to boot off the 120 unless a Mac ed 4x0 comes out.