These photos were posted by Sonny Dickson, and they confirm the supposition that the new iPad Mini won't look very different from the current model. Just about the only difference will be the volume buttons:
It will likely have a faster CPU, higher resolution camera, and (probably) a Retina display.
The photos also confirm that the new iPad will be thinner and have speakers and a screen bezel similar to the one on the iPad Mini:
As you can see in the photos, the smaller bezels on the new iPad make it easier to hold it one-handed (not that I would recommend it).
There's no word yet on the price, specs, or the ship date, but I am expecting to see these tablets on Tuesday alongside at least one iPhone. Current speculation suggests there will be a couple iPhone models launched on Tuesday, though.
P.S. There is a slight chance that the new tablets won't be unveiled this week. It's entirely possible that Apple will hold a second launch event this Fall just to maximize the media circus. If that happens then I would expect that second event to be held no later than the end of September (I'm guessing).