Apple has released the Golden Master Candidate version of iOS 4, as well as iTunes 9.2 Beta, to registered iPhone developers. The iOS 4 GM Candidate is a near-final version of the next-generation iPhone and iPod touch operating system, offering a number of new features including multitasking folders, the unified Mail inbox, rotation lock, and more. iTunes 9.2 will include a number of new features, including support for iPhone 4, the ability to sync and organize books on the iPhone and iPod touch and PDFs on the iPhone, iPod touch, and iPad, the ability to create iOS Folders from within iTunes, faster back-ups when syncing an iPhone or iPod touch with iOS 4 installed, and performance improvements. Both iOS 4 GM and iTunes 9.2 Beta are available for download now from the iPhone Dev Center. Apple will launch iOS 4 on June 21 as a free upgrade for all applicable products, including the second- and third-generation iPod touch, iPhone 3G, and iPhone 3GS; iTunes 9.2 will likely appear around this time, as well.
Update: Apple has announced that developers can start submitting iOS 4 applications for approval on June 10, 2010.