How long would I have to cancel order before shipped?

[oakland6980]

I'm considering getting up early and ordering the iPad 2 online then later on Friday attempt to get one at a store apple/best buy/target ....

My question is if I place an order early in the morning will I still be able to cancel it by the end of the day?

I know if it does get shipped i can always return it to the Apple Store (1 hour away).

Just seeing what everyones previous experiences are for, order --> shipped, times. Particularly on dates similar to a launch date.

Thanks all