A working method with full instructions can be found here:-
http://pentaxda.deviantart.com/journal/Turning-DFS-Off-224124268
For the files that are mentioned and needed, they can be downloaded here directly, if you just don't know how to compile them yourself:-
http://darky.is-a-geek.net/debug_mode/disable_dfs.zip
This may be useful for those who need to do astro-photography in a continuous way without any interruption for longer time exposure per shot.
As for the latest Pentax DSLR models after K-r (i.e., K-r, K-5, K-30, K-5II and IIs) that cannot open the Debug mode by the above method, you can try to use the PK Tether software and then load the SYSPARAM.TXT file at the SD Card Root to see if it works or not! And please let us know about your result if you really try!
Btw, today, I've just come across these photos taken by a Japanese with his EOS 60D and 10-17mm wide angle lens combo. They are quite nice, look:-
http://www.geocities.jp/tpkkagato/
(in Japanese)
N.B. Pentax DSLRs using the Samsung sensors (i.e., K20D and K-7) do not have any option to turn off the DFS (Dark Frame Subtraction) whereas other Pentax DSLRs using the Sony sensors can have the option to turn off DFS for exposure that is not longer than 30 seconds, otherwise DFS is forced to be turned on again.
Related:-
K-5 and K-r Debug Mode Unleashed, Finally!
Successfully Open the Debug Mode of My K-r! :-)