You can purchase the AP's at any time from 407-WDISNEY or the Disney web site - what you will receive in the mail is a voucher to be redeemed at any Guest Services at the 4 parks or DTD. When you get to Orlando, you go turn in your voucher to one of those locales, and your AP is activated, and begins on that day. Your AP will then be good up until a year later, on the same date. Hope this helps - it can be really nice to spread your expenses out by buying it now, rather than waiting until you get to the parks!
(It may go without saying, but if you just walk up and actually purchase at Guest Relations, you don't get a voucher - you get the AP, and it is activated instantly.)