These are idle temps. The temperature in this room has increased slightly, thus the numbers shown are slightly higher than what I quoted earlier. Load temps were in the 65-70C range on the individual cores. The diode temperature during a brief load period increased only 3C, to 61, and I did notice that the core temps once the load was removed dropped 20C in a matter of seconds. This is looking more and more like either a sensor calibration issue from the BIOS, or (though remote) a possible bad CPU?
Edit: To rule out the CPU itself, I could swap the i7 920 back in (it's sitting on my desk, right beside me). I have also flashed the BIOS to version FB, the latest available from the Gigabyte website.