Apple switched to using a different of sleep/wake sensor to trigger their "Smart Cover" in "The New iPad". This different sensor is sensitive to magnetic polarity (the sensor in the iPad 2 is NOT sensitive to polarity), so a particular model iPad 2 case has a 50% chance (or 100% or zero % chance depending on assembly practices) of working with an iPad "3"?
And I found a solution: if your case doesn't perform sleep/wake: Take a small piece of a thicker refrigerator magnet and test it. If it works, glue it to the inside right edge of outer flap, about 2 1/2 inches from the top of your ipad 2 case. A 1 inch by 1/2 inch size or smaller works best, but too small or too thin (ie magnets on back of business cards) may not have enough magnetism.