Originally Posted by PePe 
Sigma was promoting this feature on their website already a bit earlier. They do claim that also lenses with Pentax mount do have a built-in optical stabiliser.
It is still unclear to me how their system really works, what sort of performance can be expected etc. There is probably no syncronisation between the optical stabiliser in the lens and the sensor stabiliser within the body. Will be interesting to see what happens when you have them both working simultaneously!
You would switch off the Pentax SR when employing the SR on the Sigma lenses and visa versa.