If you pay by credit card, Apple will put a provisional transfer on the credit card to check that you can pay for the purchase. They hold that and will only charge at the point of invoice (that happens once the items have been shipped).
I believe - I've never not used a credit card - if you use a debit card, they can't "reserve" the funds as they can a credit card so have to take the payment right away.
Curiously enough, when I ordered my iMac 27", I'd had the machine for about 4 days before the money was actually taken.
I paid by cc on the 10th and until today my balance was zero, but my limit was £629 less than maximum, like the sum was reserved but not taken. Now my balance is £629.