While many do not even consider the iPad to be “behind” from what Apple announced in late January (early April versus late March, heh), Asian market sources already kind of “pointed the finger” at a possible culprit : Wintek, responsible for the manufacturing of the tablet's touchscreens.
Apparently, the supplier struggled to produce the components and delayed Apple's production line. Wintek denies the information, however, and says that everything is going as planned. In addition, it did not comment on specific customer orders.
I think what was more serious than the iPad's “delay” was the postponement of delivery dates for some of its accessories. Hopefully, the situation will normalize as soon as possible.