Mandelbulber uses 100% CPU while rendering. With DOF on, only one cpu is used.
Win 7 pro 64bit
12GB triple channel DDr3 1600mhz ram
Intel i7 950
It is not a bug. It is because there is very difficult to divide this algorithm for many CPUs. Only main rendering and Screen Space Ambient Occlusion uses multiple threads. That's why when DOF is rendered the CPU load is lower on multi-CPU machines.