The packaging is completely different for the iPad 3 model. The older Mag Snap 2's had no sticker on the packaging saying anything about fitting the iPad 3. They had maybe 20 each of white/black for the iPad 2 and 20 black for the iPad 3, all marked down as Clearance @ $12. Incase only offers the MagSnap 3 case in black thus far. If yours had a sticker on it saying it worked on the iPad 3 (the pic below isn't as sticker...it is part of the packaging), it was likely placed there by a blue-shirt at BB. Even Incase's official website placed the MagSnap 2 in the "does not fit" classification when they announced which ones did and didn't on launch day. So all I can figure is there was a misshapen batch of them out there as we all know the size/shape/dimensions of the new IPad are identical.
It was $40, super pricey for what it is, but having had Belkin's, Zum's and a host of other back cases that cracked and failed in the field with my iPad 2's, the Incase was the only one that held up. I don't mind paying that kind of coin for something I'm only going to have to buy once. I already have several Smart Covers from the old fleet so no issues there. I'll hang onto my Targus Vuscape just in case I need it again but I was never happy with the writing angle that case provided.
I tried fitting it into an iPad 2 MagSnap case in the store top first, then bottom and it would just pop right back off. Tried it with 2 whites and a 1 black. All the same, no matter what I did (top first, left first, right first, one corner first, with green eggs and ham, in a boat, on a moat, etc.), it would pop right back off.
The Mag Snap 3 fits exactly on my iPad 3 the way the Mag Snap 2 did on my iPad 2. The case grips the very edge of the silver past the bezel, evenly and cleanly all down the right side and at the upper and lower left corners.
Here's what the new packaging looks like.