I bought the slim case too, but Best Buy bagged up an iPad 2 instead of a 3rd gen.
Accidently opening the wrong iPad in excitement, I tested the new case.
The magnet seemed to work, I'd instantly hear a click, but followed by a 2nd click, and even more if I barely touched the cover. The screen always seemed off, but sounded like it wasn't aligned, I figured because it wasn't the new ipad.
My last iPad was 1st gen, so this is my first experience with the magnet.
When I exchange the ipad 2 for the new one, I may be tempted to test the case in the store. The seem to be a lot of issues with the magnet looking at Amazon reviews