I have both the *istDs and the K10D.
I like both of them. The K10D gives more control, using the two wheels - while the *istDs requires you to use the menu. The extra 4 MP and shake reduction have assisted me, I find that I am using SR as a crutch (same on my K20D - another story).
The K10D - if the price is right - is a really nice upgrade from the *istDs, but you can take wonderfull images with the *istDs -- hint
PENTAX Photo Gallery mostly *istDs with one K10D image. The "Aroaki / Mt Cook 1" image was printed at 20x30 and is hanging in my wifes office - came out beautiful - so much for can't print large images from 6MP.
The K10D is a very nice camera, my son preferrs the *istDs because it is smaller and lighter. I like the K10D/K20D because of the IQ and SR - although I am trying to learn how to get away from using SR as a crutch. (Have to remember to wait until the shutter has finished before moving the camera - motion blurr by bad technique is my fault, not the camera's)
If you can get a K10D for a good price - you will not be disappointed - fine machine. The K20D is even better -- more control.
The Elitist - formerly known as PDL