It appears Apple is using a new design of sleep/wake sensor in the new iPad. A sleep/wake sensor that is sensitive to polarity.
Apple decided to use a polarity sensitive switch in the new iPad 3. Some iPad 2 customers were having issues with their iPads entering sleep mode when they flipped Apple's Smart Cover around flat behind their iPad 2. Since the iPad 2's sleep/wake sensor wasn't polarity specific, it could sometimes be triggered from the magnet being positioned at the rear of the iPad. By changing to a sensor that requires the correct polarity, the problem is eliminated.
Some of the original genuine Apple Smart Covers do not work properly with the iPad 3. Sometime in 2011, Apple apparently modified the design specifications of the Smart Covers in preparation for the release of the iPad 3.