God's foreknowledge is conventionally explained in one of two ways. One is that God's knowledge of how things will be rests on his intention to make them happen so. This explanation implies that God brings about all that happens, and thus restricts our moral freedom to giving our will to it or withholding our will from it; if the same explanation is given of God's foreknowledge of which we shall do, we have no freedom at all. An alternative account is that God apprehends the past, the present, and the future all at once, being himself unchanging. If this were so, he would not apprehend change as it is: only one who is in time can be aware of anything _as_ changing.
Both of these claims seem to be a bit hasty, for different reasons. While there are people who hold something like the position Dummett gives for the first, far more common (I would think) is a looser claim that God necessarily makes some sort of causal contribution to everything that happens, and, knowing his causal contribution, knows the things. There are problems with this if it is a stand-alone position, but is rarely stand-alone, since most people who hold such a view also hold the second view as well; they are not mutually exclusive. The second claim, the reason why the eternalist view won't work, seems to be simply false, or, at best, equivocal. If God is immutable, and knows past, present, future, all at once, then his knowing past, present, and future is unchanging, but what is known in this immutable knowing is changing: change as it is. There is no problem here. Consider several parallels in this regard:
God's knowing of places is non-spatial; but what is known is spatial.
God's knowing of evils is not evil; but what is known is evil.
God's knowing of the merely possible is not merely possible; but what is known is merely possible.
God's knowing of visible things is invisible; but what is known is visible.
God's knowing of temporal things is atemporal; but what is known is temporal.
God's knowing of changing things is unchanging; but what is known is changing.
The list could be made much longer than I've made it; we would need very good reason to except time and change from the list. I've talked (well, argued) with several open and process theists on this point, and the only one who ever came close to giving me a good reason even for thinking such a reason might (on any defensible suppositions) be given was Lewis Ford, the process theist (for those interested in process theism, his book, Transforming Process Theism, is a great read, by the way); and it was a very complicated response, involving long forays into the metaphysics of time, experience, being, and knowledge. Suffice it to say, if you're not a process theist, you have no basis on which to make the exception; if you are a process theist, you might be able to provide such a basis, but it is hard to evaluate because of its complexity. It's certainly not the easy move many people (Dummett is just one example here) think it is.
The issue with God's knowledge vs. what God knows is a perfectly general issue; we could make a list for our own knowing very similar to that I gave above for God. We may be necessarily temporal, but we are not so because we know temporal things but for some other reason (e.g., because of what we ourselves are).