Recently Ulatowski has ported the game to the iPhone. While on the Treo it used some basic video registration to place the invaders around you, on the iPhone it takes advantage of the compass and accelerometer. Although it does make the alien spaceships a little wobbly, it still looks like the best game of its type currently available for the iPhone and a lot of fun:
It will cost you $3 on the appstore. More details can be found here.