If you own a copy of the Atari Greatest Hits app for iOS, you may have noticed that the most recent update promises support for a mysterious device known as the "Atari Arcade -- Duo Powered" and linking to this image and description.
The kit, put together by Discovery Bay, features an arcade stick and four buttons and will certainly provide a more authentic control experience than the existing touchscreen controls in the app -- some of which work considerably better than others.
Since the games in the Atari's Greatest Hits compilation play in both portrait and landscape orientations, the controller will support both also, connecting to the iPad via Bluetooth rather than dock connector. This is the same way in which ThinkGeek's popular iCade device operates -- by emulating a Bluetooth keyboard.
There's no word on a release date or pricing as yet, but some commentators speculate it may be cheaper than the iCade due to its simpler design.