On my P5B Deluxe with an early Conroe E6400 CoreTemp seems to be reporting 100% correctly. At 85C thermal throttling was active on both cores and it couldn't go any higher just as Intel intended. I think TAT was reporting 2C too low.

CoreTemp seems to have a problem with the E4300. If it reports your idle temps lower than ambient then I would suggest adding a correction of 15C (100C-85C) to the reading and it should be right. Not all E4300 chips are being misread but there is definitely a problem on some boards and not others.