The iCade, which communicates with the iPad over Bluetooth, offers for a surprisingly faithful arcade experience with clicky buttons and a solid joystick. They feel just like real buttons; they sound just like real buttons. It's also nice just to be able to play a game on your iPad without cradling it in your lap or laying it flat on the table. The iCade, which isn't actually a dock so much as a stand, nevertheless lets your iPad just chill out as a display for gaming, as opposed to making it pull display-and-input double duty. It's a much different experience, and one that really expands the iPad's capacity for gaming.
Ion Audio's teaming up with Atari for the first run of iCade-compatible games—Asteroids was the one I tried, a solid port of the original—though Atari's working on bringing their entire library to the iPad and iCade (those without the iCade hardware will be able to enjoy the games with on-screen controls). Ion Audio's looking to work with other developers to create further iCade-compatible games, too.
You'll be able to pick up an iCade for $99 this spring.