Pretty addicting game you've got there!
1. I know the basic code: random placement code at the bottem, add a counter for all balloons that pass a certain place, every 5 one-point balloons has 1 five-point ballooon. It's good, just add a pop sound at the "Hit Down" section of the balloon button.
2. It needs some music, prefferably changeable. And perhaps a changable background.
3. A hot-key for pausing, something like// on(release)//some code for hitting a key//stop();// then some key for resuming the game.
I can't say I'm very good at actionscript. Go to the AS:Main in the Flash forum and check out some tips on AS-ing.
Nice, addicting game.