I've been playing Warheads for a while and I think it's a great little time killer...so much cooler than missile command.
A couple of tweaks would make this game a lot more fun.
The things I would change;
Make nukes appear earlier in each round...I seem to get most of them at the end of each round. Also, nukes should destroy everything in the air.
More ammo boxes, and make them a color so they stand out! Also, give a bonus amount of ammo after each round according to how much of your base is left.