Histogram question

[Glen]

I've been told that a lot of DSLR cameras use only the green channel when 
calculating the histogram, instead of the full RGB data. Does the Pentax 
*istDS display a true RGB histogram, or not? It seems that there have been 
times when the histogram hasn't been entirely accurate for me. I got 
clipped highlights sometimes when the histogram didn't suggest any such thing.

thanks,
Glen