You'll probably see a more noticeable difference in the way the K20D records skin tones, and depending on the type of lighting, it produces a smoother almost buttery look. If anyone currently has both the K10D and K20D, it might be a great service to your fellow Pentaxians to provide better samples than my couch to show these differences in tone.
Both images shot in RAW at ISO 200 and f/4.0 with the 31mm Limited lens. I processed the images "as shot" using Camera Raw 4.4 and PhotoShop CS3. As I had to switch bodies and remount the K10D and K20D to get these shots, there's a slight difference in the framing of both images. Also, I was a little out of focus on one shot, but these samples should not be viewed based on the point of focus but examining the color values between the two images.
K10D Image
Large K10D Image jpeg
K20D Image
Large K20D Image jpeg