Back to Rear Hazcam Sol 2169
This certainly DOES NOT look like any natural rock formation that I've ever seen. Notice what appears to be a row of lights or windows at the bottom of the pie tin shaped object at the top center of the frame. It is possible that this may simply be some kind of artifact from the ultra-wide angle lens used in the rear hazcam. To view the photo on the NASA Spirit rover web site, click here.
UPDATE: I checked a number of the previous days' hazcam images, and the same object appears in the top center of the frame. That leads me to believe this is probably some kind of artifact of the ultra-wide angle lens used in the rear hazcam. It's more than likely a piece of equipment on the Spirit rover itself, IMO. But I could be wrong about that.