That's pretty cool, I'd agree that this seems in some ways like an even better promo than the 32GB + battery + charger. If nothing else, it's a chance to introduce more people to the leather backs (owners as well as people seeing the phones). People who want more storage will buy a card. But not everyone will pony up the ~$40+ for a leather back.
I do have to wonder if this is an attempt to help boost sales. Obviously that's what promos are meant for. But I mean offering a "better" promo after they've had a chance to start evaluating sales. Perhaps things didn't meet their initial goals, so they further sweetened the pot?