Several Apple news websites are reporting that the next-generation iPhone might debut on September 12, with a release coming on September 21.
9to5Mac (http://9to5mac.com/2012/07/29/apple-to-unveil-next-iphone-and-smaller-ip...), Cult of Mac (http://www.cultofmac.com/181773/the-iphone-5-ipad-mini-next-gen-ipod-nan...), and Mac Rumors (http://www.macrumors.com/2012/07/30/iphone-5-and-ipad-mini-to-be-announc...) all published stories this morning claiming that the next iPhone—as well as a new iPod Nano and the iPad Mini—could be announced as early as September 12, with a release of the iPhone nine days later on September 21.
The source for all three articles is a report from iMore (http://www.imore.com/apple-iphone-5-and-ipad-mini-event-planned-septembe...), a website with a proven track record of accurately predicting both the launch date for the new iPad and the preorder date for the iPhone 4S.
Naturally, any information provided before an official announcement from Apple should be taken with a grain of salt. However, given iMore’s recent track record, it would be reasonably safe to assume that, at the very least, the next iPhone could launch sometime in September.