iPhone: Checking the option to encrypt your iPhone when backing up with iTunes will not only increase the security of that data, it will also make restoring more convenient.
You should encrypt your backups, if not for security reasons, for a big convenience gain: encrypted backups will include your email and Mobile Me passwords so you never need to re-enter them after a restore. (emphasis added)
In case you don't already know, you'll find the checkbox option to encrypt the backup in the iTunes summary screen. Be sure you remember the encryption password!