Previewed at WWDC earlier this year and currently released as GM seed, the brand new iOS 6 will be available to the public on September 19th.
In iOS 6 iMessages and FaceTime was improved to unify a cell phone number and Apple ID and allow FaceTime or iMessage communications across Mac, iPad, and iPhone using just a number. YouTube and Google Maps integration was removed for iOS 6, but Google has already released a stand-alone YouTube iOS app. Apple is using their own maps in iOS 6.
Featuring over 200 new features, iOS 6 will be available at no extra cost for existing compatible devices — iPhone 4S, 4, 3GS, iPad 2, new iPad, and iPod Touch.