I contacted their support about this, because I was curious as well. The representative stated that the original iPad mini Defender case would be incompatible with the retina iPad mini because the retina iPad mini has a second microphone that the original did not. The new case will include a slot to account for this second microphone. Looking at the case designs, it looks like it's on the upper back of the device.
It's not clear to me what the purpose of the second microphone is (noise cancellation, like on the iPhones? Better audio clarity?), nor what having it covered up would do to audio recordings or Siri commands.
The Otterbox representative didn't say anything about the case dimensions not fitting, though. I assume it would fit. Amazon has already updated their iPad mini Otterbox Defender listing to claim that it's for both the original iPad mini and retina version.
Interestingly, the dimensions and weights for the two Defender case versions are a fair bit different. The weight of the original was criticized, but the redesigned version is about a third of the weight... but I sort of wonder if Otterbox's technical specifications on their product website are incorrect.
