At this time, a credit card is not required for all Apple iPad pre-orders.
You'll need a credit card ...
1) to pre-order the iPad for home delivery (shipment directly from Apple to you).
2) to pre-order the WiFi + 3G models of the iPad
You won't need a credit card ...
1) to pre-order the WiFi-only iPad for in-store pickup at your local Apple Retail Store
For more, refer to our list of important Apple iPad pre-order details.