I'm re-reading Robert E. Picirilli's excellent work Grace, Faith, and Free Will. He broaches the subject of Divine Foreknowledge of future events.
He's very clear on the subject, and convincing. He draws from Arminius himself and from Richard Watson, although he admits that the 19th century theologian's style is belabored. I'm not sure what is original either to Dr. Picirilli or to his sources.
In particular, Dr. Picirilli cites the simple illustration that we ourselves know with certainty specific events which occurred yesterday, but that none of us would claim that our present knowledge of yesterday's events caused those things to happen or that such knowledge limited our choices when we were faced with them. In the same way, God's knowledge of the future doesn't cause events to happen or limit the human's freedom to choose to do one thing or another.
Dr. Picirilli explains further that God's knowledge of the future does not make those events necessary, only certain. He writes, "An event can be certain without being necessary: 'shall be' (certain) is not the same as 'must be' (necessary). Some events are 'necessary'; that is, they are inevitably caused by a prior influence. Others are 'contingent'; that is, they are free, capable of more than one possibility depending on an unforced choice. Both kinds are equally certain, as known to God" (p. 37).
A Calvinist acquaintance attempted to dismiss this argument by suggesting that God wouldn't send his Son to die for people whom he knew would certainly reject him. But this probably proves too much, for God likewise would know that you will commit adultery with someone the third Tuesday of next month; does this mean that he would not bother administering grace and sending his Spirit to enable you not to fall to temptation? In some respects, there is a speculative aspect to these sorts of questions. At any rate, Jesus' death is not something which can be reduced to a mathematical equation, as if God extracted some specific amount of suffering to atone the sins of a specifc number of the elect.
Ultimately, the issue is whether or not God foreordained the future. This is an issue which the Arminian and the Calvinist need to hash out. But the discussion cannot be short-cutted by the Calvinist's appeal to God's foreknowledge. As others have argued, future events would still be certain even if the Open Theists are right and God doesn't know the future.