I have wondered since the first leaked shots of the new iPad's rear shell hit the web what the purpose of increasing the taper on the back was for. Typically Apple does not make design changes just for changes sake, they make a change for a reason. After seeing this tweet it hit me.
Apple staff says ALL ipad2 cases work with iPad (v3) !!!!— Georgia (@GeorgiaTiPb) March 16, 2012
Apple needed more battery capacity to compensate for the extra power consumption of the retina display and the LTE chip set. Apple had to increase the thickness of the iPad to accommodate this larger battery. I believe that Apples intention was to keep the design identical. In order to allow all iPad 2 cases to fit the updated model they added extra taper.
Any loose fitting case would not be an issue but tight rear shells would need some extra room to stretch and compensate for the extra 0.03" of thickness on the new iPad. The added taper gives tight case the room they need to stretch and still fit the new iPad. I love Apple's attention to detail.