Apple has just released iOS 7 Beta 2 to developers. It's now available over-the-air (OTA) and on-device via Settings > General > Software Update, and should be soon as complete download via developer.apple.com as well.
The only public-facing information about it right now is that it contains bug fixes and improvements. iOS 7 remains under non-disclosure (NDA) as Apple prepares it for public release this fall.
Update: For the first time, iOS 7 is now also available for iPad and iPad mini