It seems like it's going to happen on 12 September, the unveiling of the next iPhone.
The first buyers will get their hands on it on 21 September. This is according to www.imore.com. A smaller type of iPad will also be unveiled. There might even be a new iPod touch too, but an iPod nano seems certain.
The new iPhone screen is likely to be 4 inches diagonally and have a 16:9 aspect ratio unlike the current 3:2 ratio. The whole device will be thinner and have a new smaller connector. The new software for it - iOS6 - has already been released, so at the moment we know more about its brain than its body.
Here's a post about iOS6 I wrote earlier: http://proctele.blogspot.se/2012/06/whats-new-in-ios-6.html