My Rocketfish Hard Case rear shell does NOT fit properly on the new iPad mini. It really is a hair too small. It snaps on to the new one, but barely. The corners always slip off when you're holding it. I love this Rocketfish rear shell (i use it along with the Smart Cover). I'm patiently waiting for an officially fitting rear shell to be released.
Some manufacturers are simply slapping the "compatible with the new iPad mini" on their listings. One manufacturer in particular recently had a sale on one of their iPad mini cases, also listed as compatible with the retina mini. After corresponding with them over email I was told that the cases fit, but "not as well", thus the reason for the sale. They also realized after I asked that the cases also had no hole for the second mic.
I sense there's a lot of assumption out there that all the original iPad mini cases will fit the new one, because the increase in thickness is indeed slight. But a lot of these cases are pretty precise in their measurements. And the increase is significant when it comes to this kind of stuff. That second mic hole is also an important factor to consider.