Playing with the latest beta software for iPhone, one of iH8sn0w employees (firmware 3.1 beta 3 jailbreak utility), found that the expiry date for latest firmware is September 18.

Firmware 3.1 will be probably released 1 or 2 days before the expiration of the beta. This means that Apple might perform a release during the Apple event in September. However this event is dedicated to the music, so we cannot be sure.

The only certain knowledge is that by the middle of next month we will see either 3.1 beta 4 or 3.1 release.