Log in, agree to new agreement updates, and click the "iOS SDK beta" tab on the top left.

From the email:

iOS 5, the latest version of the world’s most advanced operating system, includes over 200 new user features, and an updated SDK with over 1,500 new APIs and powerful new development tools.

Start implementing an incredible array of new features that will delight your users, including iCloud Storage, Notification Center, Newsstand, iMessage, Twitter integration, Game Center, and more, by downloading the new iOS 5 SDK today.