Mars Defender is a space shooter with an RPG storyline, in development for the iPhone/Android. It's loosely based on the classic game Asteroids, although with proper 3D models and a large game area. Later missions also involve other spaceships fighting with you and against you. You can try out an early web-player demo here: Mars Defender Web-player demo (4 levels prototyped so far - requires installation of the small Shiva/Stonetrip web player plug-in) In the game you are generally trying to defend the planet itself or the Bova Space Station from asteroids and enemy ships; some missions also involve travelling to Phobos/Deimos and later to other locations in the solar system. The story focuses on a war between Asteroid Belt colonists and a Mars/Earth alliance.
It's now live on iOS http://itunes.apple.com/us/app/mars-defender-asteroids-space/id464571849 The Android version was live last week and is doing quite nicely in the arcade & action section: https://market.android.com/details?id=com.psychicsoftware.marsdefenderlite http://www.psychicsoftware.com/MarsDefender/