http://farm6.static.flickr.com/5161/5265493794_32fb552ee7_o.jpg
This, photo number 1782, is the first time I ever Thought something was going on. As it happens, this is a wide open shot (FA77) and it was explained to me at the time that it may be Dust on the lens showing up with the back lighting of the tree lights. I don't believe my lens is that dusty and haven't had it out of the bag since but I accepted that explanation at the time.. Maybe wishful thinking on my part. I cannot find similar with photos of that type taken with other lenses. (I was doing a different lens each day for the 12 days of Christmas game).
The photo of the guitar that I show was in fact done with live view and it's the first time I can find where there is anything remotely resembling 'the stain'. It's entirely possible it's been there all along, or just turned up. The types of photos I typically take, it might not have ever shown at all (see my flickr account linked on the banners in my sig). That is, since it's in the center, it would never show on a photo of my black dog, my dark guitars, my dark trains, etc. People pics are typically taken at around f5.6-f8 or wider so it isn't likely to have shown there either.
The photo of the Computer screen I give however was taken to demonstrate something else. In that case, the camera had only been on a few seconds and live view was not used (to answer the question about live view heating the sensor).
What you suggest about the oil splattering, over time, would make sense to me if it weren't for the fact that reports of these stains have been turning up almost since day one of the K5. My eyes aren't good enough to really see anything going on inside the mirror box. I stated earlier, that if it Were oil, I would think it could be cleaned rather than replacing the camera. I suppose it's possible that the oil, if that's the case, contaminates the AA filter to the point where it cannot be cleaned without destroying it. At the very worse, I would think a Wet clean would Smear the oil. In that case, if self done, we would be stuck paying for our own replacements. Pentax will not warranty cameras with such work done on them.
I'm willing to accept that the stains may get progressively worse with use but I'm still on the side of believing it's on the Sensor side of the AA filter. I may be proven wrong and it's an interesting discussion but at this point, I just want a clean camera. I don't want to have to avoid stopping down or having to clone that much out of my photos. That stain is too dense for me to make good use of cloning it out. I didn't go looking for this, it just turned up so I'm not a member of 'the I can't be happy with my camera club' either.