I have written a special rom for GTX Titan, which allows it to work in OSX 10.7.5 and 10.8.3.
It supports both 32-bit MacPro 1,1/2,1 and 64-bit MacPro 3,1+.
Since it involves several low-level hacks, hackint0sh user won’t be able to get it to work, sorry.
I’ll upload the rom after I fix 5 GT/s issue, so far it runs at 2.5 GT/s.
Update from netkas:
With updated nvidia drivers from 10.8.4 beta Geforce GTX TITAN works. Now for real.