Could be anything, all processors have errata, intel just discovered another one, and informed microsoft and bios makers how to avoid it. No real problem.
Probably takes a specific code sequence to even trigger it, as there havent exactly been thousands of people complaining their c2d systems crash too often.
My own system, I can count the number of crashes on 1 hand since I built it, the same week the C2D was released.
There were unfixed errata in P3 that never even got an update, and that one was faily easy to trigger, but virtually no real world programs were effected.
Doubt its a miscalculation though, that would have shown up a lot sooner imho, like the floating point bug in the original pentiums.. Was quite soon after Pentium's release it was discovered.