That's where the speakers are now, but it seems way to big for them. It has to be something else. Kodawarisan says that it is for the rumoured SD camera card slot, but it's way too big for that too. In any case, I'm very curious now.
The $US2-$US4 cases are only available for bulk sale from a Chinese company, with a minimum 1000-unit order.
Historically, Chinese manufacturers always get a jumpstart when it comes to make skins and cases for future Apple iOS devices. My guess is that they get the physical specs early from the low level operators at the factories that make Apple goods from unicorn ponies and babies. With a rumoured February 2011 and a likely April 2011 launch, the iPad 2 specs are most probably already loaded and ready to go.