Apple today released the GM version (Golden Master, that is, the final if no last-minute bug is found) of iOS 8 for developers.

Next to iOS 8 GM (build 12A365), Apple also made available the Xcode 6 GM (build 6A313) and the Apple TV Software GM (which is minimally compatible with Apple TV third generation). She also started testing Xcode 6.1 beta, build 6A1027.
The updates are already available both through the portal dedicated to developers and through the update system of iOS (in the case of iOS 8 GM) and OS X (for Xcode 6 GM).
As already reported, the final version of iOS 8 will be released to the general public on September 17, two days before the arrival of the new iPhones (6 and 6 Plus) to the market. It will be minimally compatible with iPhones 4s, iPads 2 and iPod touch 5th generation.