I think you might be confusing noise with hot pixels (which is also noise but the worst type). I have confirmed that Pentax decision to not allow K20D users to disable the automatic DFS (Dark Frame Subtraction) is because the Samsung sensor runs hot and produces a lot of hot pixels on extended exposures.

Pentax knew about the problem and covered it up by forcing DFS, in order to get this camera to market as quickly as possible. The guessed that only a minority of owners would complain and therefore the risk was worthwhile.

Bottom line, the K20D is not designed for long exposures, nor ever will be. Do not expect a firmware upgrade to address this problem. It makes me mad too!