As per the FAR's its the pilot's ultimate responsibility to see and avoid other aircraft, even under an ATC enviroment. Although it's not stated specifically, as the ultimate control of the aircraft lies with the pilot and he/she must accept/decline an ATC clearence, the consequences of the action lies with the pilot. Even if controller purposely put you on a collision course with another aircraft and the visiblity is nil so you can't see the other aircraft.
With that said, a pilot and an ATC have a responsibility to report any near misses. The pilot can usually do this, in the United States, by filing an ASRS (Aviaton Safety Reporting System) form. This system gives the pilot immunity from any prosecution, except under cases of an accident where severe damage to the aircraft or injury/death to an individual has occured.
The link to ASRS is: http://asrs.arc.nasa.gov/main.htm
. I suggest you check it out for more information and they have stories posted about various aviation incidents that pilots and other other people have reported.
- Neil Harrison