A forum post on a Chinese site, WeiPhone, contains numerous pictures of what the author claims to be the rear shell of the second-generation iPad mini. It's hard to say for sure if these are actual pictures of the next-generation iPad mini or not, but they do look convincing:
The shell does appear slightly thicker, though the edges of the device have not been chamfered yet. This could be causing an optical illusion or it could be that the second-generation iPad mini won't have a chamfered edge. If the case is a bit thicker, that would indicate that Apple is probably planning on putting a Retina display into the device.
There are still plenty of unanswered questions and these pictures are in no way verified, so take them with a grain of salt. I'll leave further speculating up to the readers, so let us know what you think of the pictures in the comments.