They put a temporary hold on the account then it charges when it ships. Debit cards will automatically remove funds from the checking account though even if it is on hold.
It's called a "Preauthorization". It means that the total amount is "Reserved" from your credit limit, i.e. if your limit is $1,000 and there is a preauthorization for $250, then you can only spend an additional $750 - even though you haven't actually been "charged" yet.
For a credit card, the good news is that it doesn't actually hit your statement until the final charge is issued (or "Captured"). So you don't have to worry about interest, etc.
For a debit card, the bad news is that once it's preauthorized, those funds are basically locked out of your checking account immediately. Apple doesn't get the money yet, but you don't have access to it either. The money is still in your account (e.g. if you have interest checking, you'll still get interest on it) but you can't access it. From your perspective, it behaves as though it were charged immediately.
Yup, Sean got it spot on. Just make sure you have more money than the preorder whether or not it is a credit or check card. You don't want those overdraft fees on your checking account.