For me, it's simple.
correct exposure is when I point the camera (in spot mode) at something that is mid grey, and it is in the middle of the histogram, or slightly below middle
I think pentax sets the metering to between 110 and 120 greyscale, there was a post about this a while back.
From then on, what you do with exposure is an individual's "artestic" selection.
If you know that the camera will produce this, you can safely go +2 stops and have full detail, above 3 stops it starts to degrade a little, and above 4.5 stops the highlights are completely blown out, A similar behavior exists below for under exposure
This is based upon imperical data obtained by plotting the histogram values as a function of F stops above and below what the camera thought was Ideal metering on a uniform surface.
Anyone who does this simple test will then be able to understand the exposure range of your camera and make judgements based upon spot metering.
I will agree however that I have found metering at times a little suspect on my *istD and K10D, if I let the camera in fully control exposure and as wheatfield has indicated, the K7D appears much better. This might be somethign to do with a change from 11 metering points to 77