Apple has announced it will strengthen device security with iOS9 by moving from 4-digit passcodes to 6-digit passcodes. This will make brute-force attacks more difficult, increasing the number of possible codes from about 10,000 to about 1 million.
Also in iOS9, when a user logs into their Apple account from a new device or browser, they must first enter a verification code, which is sent to their phone or displayed on their other Apple devices.
This allows users to confirm their own new device and it also alerts them when someone else is trying to access their account from an unknown device. When the pop-up window appears or the text message is sent, the iOS9 user will have the option to select "Don't Allow."
This new multi-factor authentication method does not use biometrics, and only goes into effect when a user is logging in from an unknown device. For more information on iOS9: http://www.apple.com/ios/ios9-preview/#foundation.