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Here's a reply from an official Apple representative (via live chat) regarding selling out (it will NOT sell out online, but it will just extend the delivery date depending on when you place your order):

You: Is there a chance the pre-ordering online will sell out?

Michael G (Apple Representative): We'll continue to take pre-orders online regardless of the availability of iPads. The shipping time might increase if the amount currently available is depleted, but we should still continue taking orders.

You: So in other words, I can place a pre-order one week from now for the new iPad, and it will still be available for pre-order one week from now?

Michael G (Apple Representative): I can't say for sure if it'll be actually in stock to be available by the 16th, as I cannot predict how many people would order the iPad by one week from now. We'll still take the order, but the time that it might take to get it might increase depending on the availability.
 
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