Interesting. One of my friends had a similar idea, except you had to aim the missiles by point your phone in the right geographical direction, and then choosing the power/angle of the shot. In his idea, there were no shields—the game was purely offensive: the closer your virtual 'missile' lands to your enemy's present location, the more points you are awarded.
The original idea wasn't too far from that... but obviously the amount of distance between you and another player drastically changes the game. The 'ah-ha' moment was automatically targeting the other player's last known location. Doing this means you don't need a level of abstraction for targeting, and it nuetralizes distance. It's the same game if someone is in the same building as you, or 3000 miles away. Granted timezones do factor in at some point. :)