I have an iPhone 4 and my wife has a 4s. We want to trade phones. Once we get Verizon to reassign the phone numbers, what's the easiest way to migrate the apps, settings and data between phones? BTW, the 4 syncs with iTunes on windows 7 and the 4s is paired with a OSX Lion machine.
This should restore the new phone to your old phone's last condition.
With iOS 5, iCloud can be a great solution.
Set up an iCloud account for each user. Each person set up his/her own account. After, go to Settings -> iCloud -> Storage & Backup and turn on "iCloud Backup" then tell it to perform an immediate backup.
After backup is complete, go to Settings -> General -> Reset -> Erase All Contents and Settings. Physically exchange iPhones, and when prompted set up the iPhone from the appropriate iCloud backup.
After the iCloud backup is restored, sync with the appropriate computer to pick up ripped media, iPhoto (or My Pictures) images, etc.