It seems that several customers ordered iPads this morning, opting for at-home delivery. For whatever reason, they later changed their minds and opted for in-store pickup. That's when things got tricky.
When requesting the switch, those customers are being told that their existing orders will have to be cancelled and new in-store orders placed. However, the employees warn, demand for the iPad has been so "overwhelming" today that they can no longer guarantee that an in-store pickup order will be available on April 3rd if placed at this late hour.
As of this writing, the online Apple Store notes that Wi-Fi iPads ordered today will be able for pickup at Apple Retail Store "...between 9:00 a.m. and 3:00 p.m. on April 3."
In other words: Selling like hotcakes.