do you have an iPhone already??
When you have the iPhone 4 in your hands, backup your old iPhone first so your backup is completely up-to-date
then, when you plug in the iPhone 4....I don't know which one will come first, activation or setup (I am assuming activation). Once you get to the setup it will basically say "yo, this is a new iPhone....but I see you've already backed up an iPhone on this computer before, would you like me to transfer all the data from (insert name here)'s iPhone? if so, choose the backup date you'd like to put on the new iPhone" which will be the most recent one you did...which I hope for you is 7/24/2010

all the backups are in a drop down box....