I think you'll find that in a few days the funds will be released, as initially Apple does a Pre Authorization to the card (credit and debit) for the full order amount of the order to ensure the funds are available. The authorization number they obtained in doing this is valid for up to 1 year, allowing them to post the charge at any time going forward, regardless of whether the funds were available still or not. If they were not, the charge would put you over limit or overdrawn.
Typically, Apple will actually post the transaction at the time of shipping. So, in the. Axe of you ordering the 3G mini, as it is not available until sometime in Nov., you should see the hold on the funds release within the next 7 days.