AnandTech has written a brief review of Western Digital's new 640GB HDD that features two 320GB platters:
While the WD 640GB drive does not fit in with the industry-standard capacity sizes, we fully understand Western Digital's rationale behind this move. This allows WD to use economies of scale with their new 320GB per-platter design and allows a natural progression up to the 1TB~1.3TB level by simply increasing platter count for each logical step. Of course, unless you use sub-prime mortgage mathematics, three 320GB platters only equals 960GB of capacity. WD engineering told us they can easily stretch the areal density of the current platter design to get to the magical 1TB capacity to match their competitors and witness the marketing group smiling (Editors Note - anyone in engineering knows just how difficult that can be).>/blockquote>
You can check it out over here.