The bottleneck is the availability of the IPS display. Apple's supplier partners simply cannot manufacture enough of these screens to keep up with demand. If Apple was using ordinary components, it would be far easier for them to churn these out. However, Apple tends to select high-quality components for their designs, often choosing new technology that is just beginning to reach mass production levels.I have been seeing them at the Apple Stores in Chicago. They were initially hard to find. I ordered mine online after failing to find them several times. You would think they could ramp up production quickly of something so simple...
They have recently increased production from 2 million units per month to 3 million, but demand is still outpacing supply (and the international rollout of the iPad isn't even complete).