Ok, I looked into this a couple months ago, more specifically at monitoring Core i7 but this appears to apply across the board to all Intel Core processors.
The information about built-in Windows Tools was conflicting. The general gist was that what was in WMI was read at boot and not updated or not there at all. The only way to get proper "real-time" readings was for a motherboard vendor to write their own WMI driver or write that ability into their existing drivers. As far as most of these posters knew, no one was doing that.
Ok here's the promising bit. Intel CPU's have a temperature sensor onboard, that you can read by accessing the MSR register directly. This is really low level, but there is an open source driver (modified BSD License) that will give you the Ring 0 access to do so, and it's signed
Openlibsys - http://openlibsys.org/
A guy in Macedonia has written a small program that does monitor the temperature of the CPU and he includes the source code with the program (GPLv2 is in main.cpp so this is open source as well).
"Temperature" - http://shefot.com/index.php?option=com_content&task=view&id=48&Itemid=37
There is some weirdness in how they report the temperature, basically it's the number of degrees away from the "thermal threshold" of your CPU. That can vary depending on CPU but a general number is available in the datasheet for each processor on the Intel website. The Core i7 920 for example is 100 (all units Celsius). This may vary for individual CPUs. All of this is explained in great detail by the author of Real Temp, another openlibsys based program here:
Real Temp - http://www.techpowerup.com/realtemp/docs.php
The extent of my coding knowledge is some limited Python so I left it there. I've been procrastinating making this post so someone who is a real software developer could take a look at it, sorry folks :-/
Anyways, hope this helps and thanks to anyone who does take a crack at it because temperature measurements would be a great addition to Rainmeter.