WD does not have a four-platter platform. The 320 GB drive they recently announced is based on their existing three-platter platform, so it's ~107 GB/platter. (I think Eugene even mentioned this on the main SR page in the news section.) If they *had* a four-platter platform, I would think they'd try to get a 400 GB product out.
I would think that this is in deed a four platters platform as the maximum sequential transfer rate announced is identical to that of the WD250.
If it had a three platter for this capacity it would have had to maintain the bit rate, which means have taken all the capacity increase by squeezing tracks closer to each other, which in turn makes little sense -but Seagate did similar things in earlier times...
So, my bet is that this is a four platter for the time being until they can increase their areal density (and sequential transfer rate), and if WD is true to form the three platter version will come unannouced.