Apple has just begun seeding iOS 5.1 beta and XCode 4.3 to developers. XCode 4.3 is available for download but may not be used to submit apps as it is still in developer preview as well.
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.
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).
Overall the release notes above are pretty uninformative. We'll update more as we find anything. If you find something, let us know!
Source: [developer.apple.com](developer portal](http://developer.apple.com)