I've just left my K20D in to be repaired for the banding problem.
I shot 112 photos with it last Thursday, more than half of which had vertical banding - in most of them I can see the banding even at thumbnail size, and all of them at fit to screen (35% for landscape). It was a nice day, about 10 degrees C, some shots were auto, some aperture priority, all using 18-55mm lens, some were very high contrast shots, others low contrast. Some that had banding were shot seconds before or after others without banding. All were JPEG (I only had one 2GB card with me so I switched off RAW, but on a previous day about a quarter of all my shots (RAW+JPEG) had banding - that day temperatures were about zero to 2C - although the previous night, in -3C there had been no banding). I have only shot 278 shots since we bought the K20D, so about a third of the total have had banding.
The retailer (a Pentax pro dealer) said he hadn't heard of the problem before.
Below is a crop of one of the photos (except for compression it is otherwise untouched) - it can be seen full size if you click on my Gallery Photos...