In parallel with the release of iOS 3.2.1 and 4.0.1 for the public, Apple has posted iPhone SDK 4.0.1 for developers. The software is essential for creating iOS apps, as it includes the latest Xcode IDE and iPhone Simulator packages, along with other related tools. Programmers with devices upgraded to v3.2.1 or v4.0.1 must install the SDK to continue development, Apple cautions.
