Yeah, that was in the Impress Japan interview with the person from Pentax. Although he said it was just his desire. That's a pretty big goof up if it really was only his desire to say that in a big interview though...
I think it could be a really interesting product, but of course it will still be very expensive and not many people could afford it. I'd like to see it cheaper than the existing 645D. Considering the loss of expensive optical and mechanical parts, I can't see why it would cost more.
If it is mirrorless, we can assume it will have full-time live-view in the form of an EVF. Otherwise it would need a rangefinder style VF, which is really unlikely.
The registration distance is around 2.8 inches (70.87mm). So I think they could get a decent size reduction by removing the OVF and mirror, but keeping the mount as is.
I think it would be a toss-up if they would change the mount or not. 3 Inches isn't too bad, but they could probably do the new mount + adapter thing if they wanted. I would assume the adapter would have full support for AF, so it would probably be an expensive add-on if they go that route.
It's certainly something that a lot of people with the cash could have an interest in. Talk about a small powerhouse of a camera...