Ok, let me answer these questions properly...
1. Intel processors have not used diode-based temperature measurement for many generations (the Intel(R) Core(TM) 2 Duo was the last that did so). Temperatures are (now) measured using Digital Thermal Sensors (DTS). There is a DTS located at the hotspot of each and every processor Core. There may also be DTS at other hotspots, such as within the memory controller or graphics cores. These DTS are used not only to report temperatures, but also to support such features as the Thermal Control Circuit (TCC), which protects the processor from catastrophic overheating, and Intel(R) Turbo Boost Technology. The processor exposes a single temperature, an amalgam of the temperatures from all of the DTS incuded, to external monitoring and fan speed control devices. This is done via a single-wire bus known as the Platform Environmental Control Interface (PECI). It also publishes a temperature threshold, called Tcontrol, that tells the fan speed control device what temperature it is to maintain (stay below).
2. The specifications for the boxed processor heatsink-fan units can vary from one processor to another. In general, the fans have an absolute minimum speed of 1000RPM.
3. The fan speed control solution on the 6 and 7 Series motherboards implements a temperature control range that is 15 degrees wide. The upper limit of this control range is known as the Control Temperature. In the case of the processor, by default, the Control Temperature is set to the processor's Tcontrol temperature (you have the ability to lower the Control Temperature, if you wish, from within BIOS Setup (or Visual BIOS if available) and/or with Intel(R) Integrator Toolkit). Operationally, if the temperature is at or above the upper limit for the control range (i.e. above the Control Temperature), the associated fan is operated at its maximum speed. If the temperature is at or below the low end of the control range, the associated fan is operated at its minimum speed. Across the control range, the speed is increased from its minimum to its maximum, following some chosen curve. The shape of this curve is controlled using the Temperature Responsiveness parameter. I have a picture that shows the shapes of the curves, but I can't seem to get this editor to let me insert it. I will have to provide later...
Let's look at an example. Most processors have a Tcontrol temperature that is somewhere between 80 and 90 degrees (there are exceptions in both directions, however). Let's suppose it's 80 for our example. What this means is that, below 65 degrees, the fan is going to be sitting at its minimum speed all the time. Let's respond to this like a FAQ:
Q. Why, when my processor temperature goes from somewhere in the 30's to somewhere in the 40's, I don't see the fan respond?
A. For our example CPU, you are not going to see any response until the temperature reaches 65 degrees.
Q. Why does it wait until the temperature is so high before responding?
A. Because, quite simply, it doesn't need to. This isn't really a high temperature Your processor could sit at its Tcontrol temperature for is entire warranted lifetime and it would not suffer any degradation due to thermals. There is obviously a need to stay below this level to ensure headroom for Intel(R) Turbo Boost Technology, however. The 15 degree control range provides this headroom...
Q. What if I want it to stay cooler?
A. You have the option of using a lower Control Temperature. You can change it in BIOS Setup (or Visual BIOS) and from Intel(R) Integrator Toolkit.
Q. I watched the fan response and saw variations in the fan speed that your explanation doesn't cover...
A. Yes, absolutely true, I dumbed down my answer to make it easier to explain. In fact, you can have two temperatures associated with each fan controller and the fan controller will respond to the temperature that requires the higher fan response. You choose the temperatures using the Primary and Secondary Temperature parameters for each fan controller. By default, for most boards, both the Processor and VR temperatures are associated with the processor fan controller and both the PCH and Memory temperatures are associated with the chassis fan controllers.
Q. I still see some variations in the processor fan that your explanation doesn't cover.
A. Yes, the processor heatsink-fan unit also has a temperature sensor in its fan hub and it will automatically speed up the fan if the temperature of the air coming into the fan gets hotter (the thresholds vary from fan to fan here too).