Which CPU?
Most of the new mobos have some kind of sensor (either built-in like the PIIs or external for others) that can be read either via the BIOS or an application like motherboard monitor... However, since the temps are only estimates, there's a fudge factor built into the monitors to attempt to get a more accurate temp. However, the temps can still be way off.
I use external probes to get case temps and approx. CPU temps. Aside from the digital indoor/outdoor ones offered by Radio Shack, there's one called CompuNurse that lets you place the thermistor where you need it (aside from smashed under the heatsink on the CPU die...

) The Abits and perhaps others may come with an external sensor to do the same and that info gets fed back to the BIOS...