Originally posted by Dr_who Pentax has said there is a difference between Film lenses and FF lenses. All new FF compatible lenses will have a larger image circle compared to film to make use of shake reduction. Of course if you turn off SR than it shouldn't be a issue. It certainly makes me wish I had the know how to do a mockup k-mount with a spot marking the sensor distance and compare image circles.
I guess we'll know more soon enough I suppose.
The sensor doesn't move *that* much with SR. At the worst, you'll just have what should be mild vignetting at one side of the image. If you're not trying to fill 100% of the frame and can live with 98% or so, then it will be fine.
Pentax officials have to think about JPEG-only shooters who will not crop or post process--they will complain of ruined photos. For anyone post-processing, it's probably not going to be a big deal.