First off, let me say that Open Theism is an offshoot of Arminianism (though many classical Arminians do not like it).
I live in Minneapolis, which is probably the hotbed of the Open Theism conflict. What is really at stake here is not God's omniscience, but whether in fact there is a future which alread exists - that can be known. If this is the case, it seems to undermine the concept of free will. It also seems to invalidate the significance of prayer, and seems to make God responsible for evil.
Open Theists argue that God knows everything that it is possible to know (including the outcome of every possible choice we make - he is therefore - Omniscient. He is also All Powerful, and Sovereign. He can work anything out according to his will. So it is not the nature of God, but the nature of free will and time itself that are in question. Regarding statements about blessing and cursing of nations and jusgment - surely there is a difference between knowing the future and making a decision about the future. For instance, I can decide to give my wife a gift tomorrow, that is a kind of foreknowledge but it is prescriptive rather than predictive and centers on my actions and not hers.
Ultimately I am not convinced by the open theist argument. I can say that they do not ignore difficult scriptures. Like all theological positions they attempt to work out a comprehensive theology that takes all the relevant passages into account. We are all more or less successful with this. In my estimation Calvinists have a good deal of scripture to answer for as well - but they don't seem to be troubled by it.
Why would anyone want to be labled as an Arminian, Calvinist, or Open Theist anyway - for that matter. These views were constructed by men - no less fallable than ourselves. They are more or less insightful, probably inconsistent in quality from one topic to the next. We should learn from them, respect them, but not live in them. In many cases it has been several hundred years.