I saw this and had to try it for myself:
profiling sensor and photon noise | darktable
It is still under development and of course the K-5 wasn't already profiled. I figured I'd have a go at it, it wasn't an easy process, but I have profiled the K-5.
Here is a 100% crop I did at 6400 ISO, absolutely no noise reduction on the left and the right was done by just enabling the profiled NR for 6400 ISO, just one click.
http://i.imgur.com/lugD7.jpg
I've submitted the profile to the developers so K-5 users in the future can have it working out of the box. If I get around to it I'll do the K100D too.