Apple has switched to using a different type of sleep/wake sensor in the new iPad 3. This new sensor is sensitive to magnetic polarity (the sensor in the iPad 2 is NOT sensitive to polarity). I have proven this in an HD video demonstration at my blog:
If you get a third-party case with a magnet in the cover that has the needed polarity facing the wrong direction, that case will work properly with the iPad 2 but it will NOT work properly with the iPad 3.
Obviously, manufacturers were not aware that Apple would be using a new and different sensor in the iPad 3. Thanks to my blog post, the word is quickly spreading and manufacturers will be fixing their cases accordingly.