Bring out your calendars and mark the date-- if AllThingsD has it right the next iPhone launch will take place at a 10 September 2013 Apple event.
While not officially confirmed, the September date makes sense. After all, Apple tends to announce new software on the June Worldwide Developers Conference (iOS 7 in the case of WWDC 2013) before launching devices running said software on the following September.
However don't expect anything too revolutionary-- according to the rumour mill the next iPhone might even be called the iPhone 5S, meaning it will be mostly identical to the iPhone 5 but with a few spec upgrades.
Other sources suggest the new iPhone will carry at least one interesting feature. According to 9to5Mac a folder called "BiometricKitUI" lies in the iOS 7 version 4 beta, with references to users swiping a sensor on the home button. Maybe the next iPhone will carry a fingerprint sensor of some kind?