Satoru Iwata may say the DS's successor will be motion sensitive, but the team at Engine Software has beaten him to the punch, creating full motion sensitivity with no attachments needed.
To demonstrate the technology in action, Engine Software has released a video of TILT, a prototype marble-rolling simulation. Check it out below.
The developers are keen to stress that this is just a prototype to demonstrate the potential uses of the technology, and that tilt detection is just one of many possible implementations. It's apparently already being put to use in a DSiWare game, due for release this year, though sadly there's no other details forthcoming. However, it does prove the tilt sensitivity is completely software-driven, a frankly incredible achievement for any team.