There have been some questions about whether you need to be a paid developer to get the SDK. The answer is no, at least at the time I wrote this, there’s still a non-fee option to register.  Refere to this post, https://permadi.com/blog/?p=17, for more information.

Note: Registering as a Free Developer enables you to download the SDK after a simple email verification process.  Registering as a Paid Developer might actually take a longer verification process.

Some of these links are not obvious on the apple site if you’re searching for the ‘iPhone SDK,’ so here are some links to make the options easier to find:

• iPhone Dev site link (below you can see it still says you “sign up as a free Registered iPhone Developer today”).
  http://developer.apple.com/iphone/

• Link to the registration form:
  http://developer.apple.com/iphone/program/start/register/

• Link to XCode:
  http://developer.apple.com/technology/Xcode.html