The motherboard temp is from a sensor mounted on the motherboard in between the ram and the cpu. I assume they rightly wanted to keep it away from the gpu area (my 8800gtx gives off quite a bit of heat).
There is no sensor on the nb or sb that I am aware of (and I've tried just about everything). The best thing to use is an electronic thermal probe.
Some of these controller panels have up to 4 integrated thermistors. A few are in the $30-45 range