In my day to day job, im currently building a set of 18 gambling games with Flash for a client. The final game on my list is Bingo and i spent Friday coding a Bingo machine that will eventually contain 75 coloured balls inside of a sphere that are mixed up by a fan. I am rather pleased with the final effect and havent shown any of my Flash Experiments on this site for a year or more, so starting as i mean to go on, give it a peek.
So, what we have is collision detection within a circle, circles bouncing inside of another larger circle at the correct angle with gravity, gravity on the balls pushing them downwards to add to the floating effect and an updward force which is a fan that repels a ball the closer it is to the fan.