This Japanese user experienced the same problem with his silver K-5, but he got a replacement which was fine.
http://d.hatena.ne.jp/hisway306/20110324/1300971454#seemore
Although many of his photos were taken with the 31mm, I noticed that he was using a few Pentax DA zooms and a large SIGMA zoom as well.
After this Japanese user got his replacement, he tested with his 31mm to ensure the image quality was the same as the one before, but I'm not sure if those dusts would come back if he started using his zooms again. Will be interesting to see if he reports the dust problem in the future. Unfortunately, I don't know Japanese...otherwise, I would ask him to test with his large zoom lenses.