As they say also on the apple web site http://developer.apple.com/iphone/
The iPhone Dev Center provides access to technical resources and information to assist you in developing with the latest technologies in iPhone OS. Log in with your Registered iPhone Developer Apple ID and password, or sign-up as a free Registered iPhone Developer today
you need an account to access iPhone SDK 3.1.2 and additional technical resources and information with your Registered iPhone Developer Apple ID and password, or sign up as a free Registered iPhone Developer today.
Knowing Apple has such a big repository your application should maybe bring something new.