The slime balls are clearly going to be for creating an idol, allowing you to pray to Notch , Ak'golyth, god of the undead , Ssssssssteve, watcher of the creepers , Sneaky Crawly *******, king of the spiders , or Bloo'ploop, mother of the slimes. , each different person giving you different blessings
But you will not get any slime balls from us, as we will grow into the size of trees and swallow the world whole!
WE ARE SLIMES. WE WILL KILL YOU.
Dibs on praying to Ak'golyth.
EDIT: Actually, I might pray to Notch. You know, being faithful to an undead god has its rewards and everything, but I mean... Notch might bless you with updates.
Perhaps slime bals are used to produce acid. A material that looks like greenish water but does non-fire damage. If ghasts are immune to fire, then lava traps don't work so well, so maybe acid is one (two) of the new block types.
Also, it's possible that if it were acid then, and might be immune, but other stuff wouldn't. Or maybe not.