It was inevitable that Apple would put a Retina display on their iPad Mini, but as rumors have long pointed out, supplies of the iPad Mini could be tight because of a shortage of the 7.9-inch Retina display that it uses.
This seems to be confirmed at Apple's recent fourth quarter earnings call when Tim Cook said that "it's unclear whether we will have enough of the quarter or not". He further explained that although he knew how much Apple would be able to make, they would not really know demand until they start shipping.
On the flip side, supplies of the new iPad Air is expected to be plentiful at launch. Reports indicate that Apple will accept requests for "Personal Pickups" of the iPad Air at launch, which is unusual since the company did not offer this service for the iPhone 5C, despite the phone being readily available.
Source: Mac Rumors