:) But somebody could say the same thing to you that you were agreeing with TLH on. "Prophecy is prophecy." I don't mean to poke fun or anything, but the text itself is very clear. God sent Jonah to Nineveh specifically to prophecy to them that they would be destroyed in 40 days, and even when the king told the people to repent, it was with no guarantee that God would go back on what was spoken.
6 For word came unto the king of Nineveh, and he arose from his throne, and he laid his robe from him, and covered him with sackcloth, and sat in ashes. 7 And he caused it to be proclaimed and published through Nineveh by the decree of the king and his nobles, saying, Let neither man nor beast, herd nor flock, taste any thing: let them not feed, nor drink water: 8 But let man and beast be covered with sackcloth, and cry mightily unto God: yea, let them turn every one from his evil way, and from the violence that is in their hands. 9 Who can tell if God will turn and repent, and turn away from his fierce anger, that we perish not?
There's also the issue of the next few verses saying that God "repented of the evil." If the Lord didn't change His mind about what He planned to do, then what would He have to "repent" of?