One of the developers, of the application which can jailbreak beta firmware 3.1 version 3 called iH8sn0w, discovered that the expiration date is September 18th. This means Apple is either going to release a new beta version or launch the official 3.1 firmware.
Several reports have claimed that iPhone Firmware 3.1 will be released somewhere in September, so it’s pretty safe to assume the official 3.1 firmware will be launched just before the latest beta version ends. If no new version, official or beta, will be released by Apple before the 18th, beta testers cannot use beta version 3 anymore.
Also, Apple usually launches the official firmware one or two days before the last beta version ends. This means we can probably expect the 3.1 firmware on September 17th.