I've had that same thought before, and it
would be cool if you could use a stabilized lens on a stabilized-sensor body and get some kind of super-stabilized multiplication effect, but unfortunately that is not that case. I don't know the technical reasons, but maybe somebody else on here can explain why it doesn't work. Perhaps the lens OS would by "zigging" while the sensor OS would be "zagging".
Even so, it is good to see OS lenses becoming available in Pentax mount. I own the Sigma 50-150mm and used to own the Pentax 55-300mm, and my experience has been that my K-x's stabilization is not stellar at longer focal lengths. I seem to remember my Olympus doing better. And some of the newer OS implementations in lenses are reported to be very effective.
On a related note, I recently wrote Tamron to see if their new 70-300mm VC lens would be available in K-mount, and I received a reply saying there were no plans for a Pentax release. Bummer, since that is supposed to be a very good lens optically and the VC is supposed to be very effective.