iOS' Encryption Policy
I don’t think many people understand how strongly your data is protected by your iPhone’s four digit PIN. Since the 3GS, itt isn’t just an artificial software barrier to stop nearby prying eyes.
Your PIN protects your data as well as access to your device. Until your PIN is entered, the data on your device is encrypted and unreadable.
However, as every possible four-digit combination can be brute-forced in under 15 minutes, the premise of encryption is only superficial, when using the default PIN code lock. If you want true security for your data, go into your Passcode Lock settings and choose an alphanumeric string. It only takes a seven character password to make brute-forcing take several years.