Arminianism is most commonly thought of as the belief that although God knows the future, God also allows free will in future actions. In this belief, adherents are able to maintain both the Classical attributes of God (such as Omniscience) while also maintaining the Classical attributes of man (Free Will). See the Church Fathers on Free Will.
Open Theism is the belief that God is free to choose not the know the future. In order to introduce Open Theism to people, I often ask: “Can God do anything?” and then follow it up with “Can God choose not to know something?”. When it is put in the manner, people are forced to weigh their conflicting beliefs about God. Is God free to learn new thoughts, create new things, locate Himself where He wants? If not, is God all powerful? Open Theism is about if God is free.
Critics and some Open Theists describe Open Theism as the belief that the future is not set. To me, that is a strange way of defining the issue. After all, isn’t the primary purpose of the Bible to tell us about God? Isn’t the core issue about who God is? I am not an Open Theist due to philosophical reasons or because I am particularly attached to philosophical notions about the future. I am an Open Theist because I believe the Bible is the source of our knowledge about God and the Bible does not present God in the Classical image.
The difference then, between Arminianism and Open Theism primarily comes down to how attached each are to the Classical attributes of God. Arminianism wants to maintain the attribute of Omniscience while trying to maintain the truth of select Old Testament stories about God (in relation to His emotions and appeals to man for repentance). They spiritualize texts that describe God’s surprise about the future, God’s reliance on others (such as when God asked angels for advice in 1 Kings 22), and God’s repentance. They hold as literal God’s emotional outbursts, God’s unending appeal to His people to repent, and God’s blamelessness for evil.
Open Theism, on the other hand, discards the traditional understanding of Omniscience (some Open Theists try to redefine Omniscience in order to maintain use of the word). The philosophical contingent of Open Theists maintain God is more righteous and better understood as Open. The fundamentalist contingent of Open Theism holds that all Old Testament stories about God are true, regardless of the implications. Myself, and select other Open Theists, take it a step farther and claim that if God wants to not know current events, that is His prerogative. For example, if God does not want up-to-the-minute updates on the wickedness of Sodom, He does not have to have it. If God then wants to go see if Sodom is as evil as He hears, He can go do that. There is no alternative figurative meaning to Genesis 18, in which this exact event is described.
Although there is a large contingent of Open Theists that do not take the Bible literally (they do not maintain that the face value communication of the writers to the reader always reflects reality), the more that someone accepts the Bible as literal, the more they have to affirm Open Theism. The case of Sodom being exhibit one.
The difference between Open Theism and Arminianism fundamentalism is how willing one is to spiritualize the text in order to maintain a Classical understanding of Omniscience. The difference between Open Theism and Arminianism spiritualists is how one values God’s interactions with humanity.
reality is not optional 
“Open Theism is the belief that God is free to choose not the know the future.”. Only some Open Theists would take that approach. Most (I agree the numbers are debateable – but at least a significant portion) would take the view instead that the totality of the future is not knowable (by any being including God). Hence, it wasn’t a choice by God to not know, it just isn’t knowable in fixed terms in its totality. Similar to me saying ‘God doesn’t know the colour of my daughters hair. Why? Because I don’t have a daughter. It doesn’t depreciate God’s omniscience to say He doesn’t know. It is impossible to know the colour of someone’s hair who doesn’t exist. Similarly if we believe the future is at least partially open to possibilities and in that portion of the future those choices haven’t been made yet…..well, it is illogical to say that God or anyone knows that future in fixed terms – being they aren’t fixed by their nature. Hence, the real debate isn’t about the content of God’s knowledge but rather one’s view on the nature of the future.
Sir,
I believe you are right. There is a contingent of Open Theists that do claim what you say. Two things:
One, God could know the color of your daughter’s hair if He forced a daughter upon you. The fact that God does not force things like that upon us is Him actively choosing not to know things. God could, for all intents and purposes, control our bodies like robots. Then He would be able to know the future with precision. How does God chose not to know the future? He creates free will beings. Why does God create free will beings? To enter into a genuine love relationship with man. The reason God does not know the future is because He chooses not to know it. This is a good description of Open Theism that puts Classical Theists in the defensive, because now they are focused on their conflicting views about God.
Second, the fundamentalist wing of the Open Theist movement (for which I am speaking) tends to try to get their description of God from the Bible. The fundamentalist Open Theists see the Bible as our source of knowledge about who God is and what God does. The Bible does not describe God’s knowledge as knowing all possible future paths or knowing everything possible that can logically be known. That is an imported doctrine onto the Bible, the purpose of which is to retain Platonistic sensibilities about God. While it may be true, if it is not clearly delineated in the Bible then we should not be using it as a definition of Open Theism.
When the debate shifts from “who is God as portrayed in the Bible” to “retaining theologically charged attribute for God”, Open Theists lose. It may feel nice to say God is “omniscient”, but that is not a Biblical word or concept. All it has gained the Open Theist movement is derision, and a shift in the debate to semantics instead of what the Bible tells us about God.
You can see a good overview of the issue here:
http://kgov.com/bel/20070125
This was also a topic in the Battle Royale: Does God Know the Future debate:
http://www.theologyonline.com/forums/showthread.php?t=21711
I guess statement that God chooses not to know things implies that they are ‘out there’ to know. I haven’t seen anything in the Bible that God deliberately holds off his own knowledge of things. It seems much more straightforward to say that part of the future isn’t out there to know by anyone.
Yes, God is into free will. Hey allows us to make choices. But that is a different thing that saying that God deliberately blocks Himself knowing things that are out there to know.