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're supposed to choose between them. Optical stabilization is arguably more reassuring at long focal lengths because you don't get to see the camera shake.