If God is unable to change, why did he (after the Flood)? If God is All-Knowing (as I've heard) and unable to change, then why did he change his mind after the Flood?


  And the LORD smelled a sweet savour; and the LORD said
  in his heart, I will not again curse the ground any more for man's
  sake; for the imagination of man's heart is evil from his youth;
  neither will I again smite any more every thing living, as I have
  done. (KJV, Genesis 8.21)
When we talk about God's attributes we can't assume He has the same characteristics as the human beings, because we have mortal bodies, limited languages and understanding and a sinful nature, while God is infinite, has a 100% sinless nature, infinite understanding, etc. So, when the bible talks about God's changes of mind, is probably because the author (Moses, for instance) couldn't express in a better way what he was receiving from God, using a limited language as the Hebrew.

It is an interesting question anyway.