One of my favorite iPhone games, Astronut by The Iconfactory, was released on the iPad earlier this week. In 24 levels fully optimized for the Retina display, you can enjoy Astronut's well-known game mechanics (a floating astronaut jumps from planet to planet avoiding enemies and obstacles) with controls reworked for the device's larger display.
Graphics and sound effects aside (they are both top-notch), I am liking Astronut for iPad even more than the iPhone version because of its control scheme. Astronut for iPad plays in portrait mode, with buttons to jump and activate boost placed at the sides of the screen where your thumbs typically are. This is extremely intelligent game and interface design. But there's more: you can use Astronut for iPhone as a remote -- this has allowed me to prop my iPad on a stand in my living room, and enjoy the game with my friends, sharing the iPhone as a controller.