More details here:
http://www.techpowerup.com/170835/Ge...-Version-.html
1152 shaders and a base clockspeed of 823MHZ with 888MHZ boost clockspeeds. It appears that the GK104 is used.
However,this picture of a supposedly a retail GTX660 PCB has been leaked:
http://wccftech.com/nvidia-gk106-bas...ti-pcb-leaked/
It appears to use the GK106.