After unveiling the radically redesigned iOS 7 and some of its marquee features, Apple announced availability info at WWDC. Developers will be able to get their hands on the first beta of iOS 7 later today, and the public release will happen later this year.
Apple typically issues several betas to developers in the months leading up to a new iPhone announcement. Expect that to happen again this year.
iOS 7 will run on the iPhone 4 and up, iPad 2 and up, and the 5th gen iPod touch.