It will probably depend on your address. If you are getting a BTO PB it is probably not worth it. 2-3 day shipping(fastest option available) for my iBook cost $18. It shipped 7/29 and it is not due to my door until 8/5.
I ordered my new Mini with Overnight hoping to get it the next day. Apple missed to cutoff time and Fedex didn't enter it into their system until the day after, which was Friday, so I didn't get it until Monday (yesterday). So no...IMO it's not worth it
I live in NYC. last thursday I ordered an ibook (stock), and it came under 24 hours later. i guess it depends on where you are located and if Apple has a warehouse near your location that has the computer you order.