An Open Theist would likely say that God knows everything that there is to know since the future does not yet exist. However, God knows what He intends to do and is actively working to bring about His will.
And Open Theist will likely say that God does indeed know all things "right now" and knows what He intends to do at certain times in the future. The God of the Open Theist is much more interactive than the God of the Calvinists. In both views that people actually hold (not the caricatures that the opposing groups tend to make of each other), God is Lord of Creation and is in control of everything He wants to be in control of. Calvinists tend to think that God maintains tighter control. Open Theists tend to think that God leaves an enormous amount of room for human input while still bringing about His will and purposes. For instance, the Open Theist view of prayer is a little easier to understand since they think that God may indeed change His plans to accommodate our righteous requests. Calvinists tend to see prayer as humankind becoming conformed to God's established will and purposes where our desires have nothing to do with how God will act - our desires are being changed to conform to his will.
I think you can actually make a pretty decent case for Open Theism from the scripture, but in my opinion, the fatal flaw of Open Theism is the issue of the nature of time. The Open Theist view of time (at least from what I have seen) is extremely simplistic and is not much more than the concept of sequence. One thing happens, then another and another. However, Einstein's theories of time (many of which have apparently been validated through discrepancies in the passing of time based on acceleration and other forces), undermine their simplistic view. I have quite a bit of sympathy towards Open Theism (and know some of the public proponents of Open Theism), but I cannot accept it myself. However I am in occasional dialogue with an Open Theist and have posed the issue of time to him. He does not have an answer.
think the main issue se ms to be the concept of what "being in the Present" would mean for God.
Open Theists seem to hold to that meaning God would knwo everything possiblt up to"right now", but that he would learn more as the future actually compes to pass...
Biblical Christianity would take that to mean that God in always living in the "right now", as he sees all things as already happened, no past/future to him , as he already knows everything already as done, as he is outside time, as the Alpha/Omega!