a discussion about this can be found on :
http://www.minecraftforum.net/viewtopic.php?f=1016&t=72802
They cant beat any block that is ore put together like a diamond block they can easily break things like sand and dirt try to use materials that are harder to break those are the ones that dont break to ghasts :3.
If they can't break cobble/smoothstone... Why in the world did I spend the days leading up to the update farming obsidian? >_>