I can't speak for all cases, but I recently asked an Otterbox representative about their Defender series cases. They said that the original Defender would be incompatible, but they didn't mention the device dimensions. Instead, they said that the retina iPad Mini has a second microphone that the original case would cover. Their upcoming, updated Otterbox Defender case will supposedly include an opening for this microphone.
I don't know the location of the second microphone; based on the Otterbox Defender images, it looks like it might be on the upper back of the device (the original microphone was located at the top center). I don't know how audio recording and Siri performance would be affected by having the secondary microphone covered up. Either way, it's something to consider when looking at using cases designed for the original iPad Mini with the retina version.