I spent a few years in optics (for an aerospace company) then I became a quality engineer for a medical device company that makes optical devices. The subject of flatness is a very serious one in the world of precision manufacturing. All proper metrology labs have at least one lab grade granite surface plate that is calibrated to be "flat" and more importantly, "repeatable" as specified by Federal Specification GGG-P-463c. What does that mean? Here's an article that explains it in great detail:
http://www.qualitydigest.com/aug03/a..._article.shtml

What does that mean to us razor owner/honers? Realistically not much. While we may strive for perfection, the world in which we live is so full of variables that our stones and razors are probably moving more than .001" over any given twenty-four hour period while sitting on a shelf. I have to agree with Kaptain Zero when he says that we can be happy flattening our stones with wet/dry sandpaper on a piece of glass.