Demand for Apple’s own accessories to the iPad seems to be overstepping their supplies. The dates for actually shipping the items ordered on the Apple online store are shifting towards May. At the time of this writing only the USB Power Adapter actually was showing May as the shipping forecast, but most accessories where showing either as mid or late April.
An article posted earlier on Electronista indicated that the Keyboard Dock was showing a forecast of May as well, but that has changed and it now shows as late-April. This would suggest that Apple might be having some success in expediting the availability of high demand accessories in greater quantities as it might reflect a change in demand for that particular accessory during the day.
It seems clear that accessories are in high demand for the iPad and that could mean limited or no supply of certain accessories in the physical stores when the iPad goes on sale on April 3rd.