I saw a discovery channel special about the battle at Troy.  The archeologist found small piles of sling stones.  They said you could tell that the stones were used over and over because of the little marks in the ends of the stones.  Apparently the Trojan defenders were lobbing sling stones down into the Greek attackers.  They showed a reenactment shot of a few guys slinging stones in an underhand motion.

They talked mostly about the seige engines of the time and even referred to the trojan horse as a seige engine.  The section about the slingers was brief but you so little of slinging on TV that it was pretty cool.

That's the best part about a sling; you don't NEED to penetrate the armor to inflict damage. Even if the Greeks were wearing bronze armor, the concussive force of a professional slinger hitting you with a direct shot would probably have one hell of an effect, and that's not even considering what a shot to the helmet (assuming you had one) might do. Then again, a sling is a great harrassment weapon, good to keeping something like a formation of hoplites and their shields busy while the javelineers and heavy infantry hit them from the other direction.