Intel used their best sensors on the Core i7-900 series. Many of the 45nm Core 2 sensors were horrible and had a variety of issues so they spent a few more pennies for the original Core i7. The newer socket 1156 Core i7 are not as good and the 32nm Core i7-970 / 980X aren't close to accurate at idle either. No reports on Sandy Bridge yet but they'll likely be far from perfect like most of Intel's sensors.
At least they concentrate on making great CPUs. I'd rather have that than perfectly accurate temperatures any day.