Actually, to clarify a bit (and it's always nice to bust Flash's balls) - ATC does not use IFF. IFF is Mode 4, and what ATC uses is primarily Mode 3/C. Most collision warning systems use Mode S.
Radar is not vital for collision avoidance, it's just another piece in the puzzle. If you're IFR, ATC uses Mode 3/C or Position Reports - and if you're VFR, it's see and avoid.
Okay, you got me, I used the wrong term. I can't believe I was actually wrong for once!

