The current standard material for ping pong balls is nitro-cellulose which is extremely flammable. This is the reason ping pong balls can not be shipped via airplane without special fireproof containers and are therefor usually shipped by boat. Hence, there are new plastic balls which are just becoming available which will one day be the only balls made. So far, the early word is that the balls definitely sound and play differently. I believe it will be a few more months before the manufacturing process is perfected.
There is also plans for a 1/2 orange 1/2 white ball which should help all players identify the spin on the ball a bit easier.