I think the problem with quoting Cpu temps in general, let alone with the C2D is there are many variables which people dont state.
Firstly regarding the C2D, there is a very informative article over on tomshardware detailing all 3 sensors on the C2D.
Regarding the Asus P5B, if u enable PECI in bios it reduces the reported temperature (tcase) by a significant margin, in my case from 41 down to 21C.
Basically TAT reports the temps of the thermal diodes embedded in each core (tjunction), wheareas tcase is the temp of the thermal diode between the cores. Motherboards read this temp (tcase), and apply it to a set of tables which are in the bios, to estimate the average temp of the heatspreader.
This all sounds rather stupid to me as this temp is entirely dependant on what the manufacturer decides, and not on the real world temp of the cpu.
Then of course there is ambient temp, cpu speed, heatsink, etc etc.
Currently my Setup is a C2D 6600 @ stock w/stock heatsink
Ambient = 22C
Core1 = 28C
Core2 = 29C
tcase = 21C
This is with PECI enabled in bios
Hope this helps