Preview at WWDC likely in June, followed by September launch.
Apple Begins Seeding of iOS 5.1 Beta, Xcode 4.3 to Developers
Apple has seeded developers with the first version of iOS 5.1 Beta and Xcode 4.3.
iOS SDK 5.1 provides support for developing iOS applications and includes the complete set of Xcode tools, compilers, and frameworks for creating applications for iOS and Mac OS X. These tools include the Xcode IDE and the Instruments analysis tool among many others.BGR provides some release notes which presently list some known issues but are otherwise unrevealing.
With this software you can develop applications that run on iPhone, iPad, or iPod touch running iOS 5.1. You can also test your applications using the included iOS Simulator, which supports iOS 5.1. There are two Xcode iOS SDK 5.1 images, one for installing on a Macintosh computer running Mac OS X 10.6.7 (Snow Leopard) or later, the other for installing on a Macintosh computer running Mac OS X 10.7 (Lion).