In
another thread here it looks pretty clear that this issue is occurring in those situations where the NVidia card is simply shut down and the computer is running on integrated graphics, MacSpiller.
No doubt iStat will be updated to accommodate the switching graphics machines before too long (if it hasn't already)
Another more comprehensive monitoring program, "Hardware Monitor" in such situations, simply identifies the same sensor as "disconnected". Its author, Marcel Bresink , explains such situations as follows at
http://www.bresink.com/osx/216202/issues.html, after having pointed out earlier that some other monitoring software gives spurious results when sensors are not actually available:
+In this case, you must be using a Mac capable of switching between two alternative graphics solutions, for example a pair of an Nvidia GeForce 9400M and a GeForce 9600M GT, or Intel HD Graphics paired with an Nvidia GeForce GT 330M. It is the correct and expected behavior to only show the sensors which are currently online. The sensors of the GPU which is currently offline are powered down and even cannot be detected.+
If you run iStat alongside Cody's useful little app for deciding which Graphics are in use (
http://codykrieger.com/gfxCardStatus/ ) and compare your GPU results with the temperature figures then it should be easy enough to see if this is the cause of the strange number you are receiving.
Cheers
Rod