There's something to be said about attempting to replicate the old-school arcade experience, as Ion attempted to do with the iCade, but sometimes you need something a little sexier. This is the Atari Arcade for the iPad.
Teased in the Atari's Greatest Hits app in iTunes, the Atari Arcade is essentially an iPad stand that adds four arcade buttons and a joystick to the device, making it perfect for playing the sort of games you'd find in Atari's greatest hits catalogue. The official page for the device mentions Asteroids (which only used buttons), Missile Command, and Centipede (both of which used a trackball).
The Atari Arcade is listed as coming soon, with no price specified. Odds are it'll be cheaper than the iCade, but not nearly as nostalgic.
Atari Arcade Duo-Powered Joystick [Discovery Bay Games - Thanks Carlunga!]