They generally can't be retrofitted onto existing planes, so airlines have to wait and order them on new ones.

FORBES: Magazine Article

Thus, anyone looking to purchase the device must either order on-line (and wait 2-4 weeks for delivery) or hoof it on down to your "local" Apple store which could be a state (or two) away.

ENGADGET: Can Apple meet iPhone demand?

But if I have not convinced you, please wait a moment before getting on the phone to Portugal to order your T-shirts.

BBC: Bruce Forsyth in 1971