FiringSquad has an interesting report from a Microsoft GameFest panel on controllers and accessories for the Xbox 360. Chief among the revelations at the panel is the development of a USB adapter that will let gamers use their Xbox 360 wireless controllers on a PC. Price and release details weren't discussed, but FiringSquad speculates that a fall release is likely.
[Update 1: Apparently the wireless receiver is old news, as Microsoft announced the peripheral at E3. My bad.]
Microsoft hardware chief Rob Walker also mentioned that they had been testing prototypes of a motion-sensitive controller for the Xbox 360, but said that testers complained about fatigue after extended play. Despite this, Walker said they still haven't totally written off the idea of a motion-sensing controller for the Xbox 360 in the future.
Check out the full article for more tantalizing hints at potential Xbox 360 accessories, including a larger hard drive and a controller with a QWERTY keyboard.