Even though it has been strongly argued that divine foreknowledge doesn't negate free will, allow me to ask the question another way. How could God know our decisions if they are truly free? To know the outcome of something is to imply contingency (and determinism). To put it another way, if a third party can know the nature of an individual then that individual cannot be the author of his nature.
The compatibility or incompatibility of divine omniscience and mortal freedom interests me a lot, although the concept of the "author of one's own nature" strikes me as relatively unclear and probably not that useful for investigating this. Sean sketches out one answer that may be satisfactory to those who believe that free will is compatible with all of one's choices being determined by preceding events. I'm not sympathetic toward this sort of "compatibilism," and I've never been persuaded by any argument that free will could exist a world where there could be an omniscient God. So, my short answer to your question is that I think that omniscience does negate free will. (My own position is an unpopular one, and as Sean suggests a lot depends on exactly how you define the key terms used in the original question and in the answer that I just gave....)