Atari is partnering with Discovery Bay Games to create their own iPad arcade adapter, its called the Arcade Duo-Powered Joystick. It does a nice job of letting you play Atari Greatest Hits app with real arcade controls. The device is the second arcade adapter for the iPad after theiCade that was created by ThinkGeek.
The Atari Arcade Duo-Powered Joystick appears to be very similar (functionality wise) to the iCade. However, the iCade carries a mini arcade cabinet design while the Atari Arcade Joystick offers a more open layout. The device simpler design looks cheaper than the iCade which is priced at $100.
You can download Atari Greatest Hits app and relive your childhood moments. The device will be available soon for about $60, and will be sold in major chain stores in the US such as Target, Toys R Us and Walmart. I will update you if its going to be sold online.