updated 02:42 pm EDT, Fri June 13, 2014
Function will require deliberate support
Apple's iOS 8 will make it simpler to log into apps by pulling usernames and passwords saved via Safari's AutoFill feature, beta users note. A given example is Facebook; if login information from the website is already saved in Safari, people will be able to simply tap on the appropriate login when loading the Facebook app. Developers will, however, have to enable the function by adding a file to their website and a new entitlement in their iOS app.
At that point, a pair of APIs should facilitate requesting credentials from Safari and showing them within an app. The system should even be able to work in reverse, saving logins to Safari and iCloud Keychain.
The iOS 8 update is due to launch sometime this fall, presumably shortly before the release of the iPhone 6.