Pre Order - Reserve
I think all Apple retail proceedures aside, we can all logically agree there is a difference in the two terms. How people decide to use them or popularize thier use on the Internet with todays e-commerce is another.
Logically though, Pre Order is to Pre Order a product or service, await its availability and be charged according to US or Federal laws that says the customer paying can or will not be charged until said product or service is shipped, delivered or rendered. It's a Pay for something now get it later type of reservation.
Reserving on the other hand can be assumed to be the reservation of a product or service and is closely related to pre ordering, but can hold a difference when an item is not in stock or available inventory and needs to be reserved for consumption once the customer is ready for it.
"To reserve a table for two at a restaurant, or tickets for a show."
These items are there, ready for use, however the consumer is not. Afterward, just not at the moment they are reserved or at a pre determined time, the consumer is ready and the goods can be delivered upon.
(Think of how a claim check used to work or still does for a retail store that was out of a sale brochure item. You submitted a claim ticket for a later delivery to the store and paid for the item upon pick up of the reserved item then.)
This is how I see it.
When you apply this thinking to the iPad pre orders.
It seems as with pre pay (most often holding a charge for up to 30 days) for final shipping of the iPad once it's available to ship. So you hold an order open as a reservation and pay when they ship it.
Reserving it will mean, once the iPad is available you either visit the physical store and pay for the reserved item at pick up or you claim it online via e-commerce and pay at the time of shipping.