PomPomGames recently published a sample video of their upcoming iPhone game port for Space Tripper. Space Tripper is an old PC side-scrolling shooter that was released in 2001 which was said to be inspired by Defender. Gametunnel describes the original PC version:
The concept is fairly simple. You control an advanced space craft that could be more of a hoover craft as it doesn't actually fly, it instead floats just above the ground. The ship is armed with two types of lasers. The blue laser is really your power blaster. The red is weaker, but it has a fairly wide range to it. You swap which weapons you are using by clicking a button on your joystick.
The big key to the game actually lies in the ships ability to switch directions at a seconds notice. At the touch of the button, the ship will spin and flip around, allowing you to fire at those cowardly enemies sneaking up from behind!
PocketGamer says the iPhone version is not expected to arrive until "early next year."