Apple released iPhone SDK 3.1.2 on Thursday Oct 8, the release email sent to iPhone App Developers by Apple states this:
iPhone SDK 3.1.2 is now available on the iPhone Dev Center. If you have updated your iPhone app development devices to iPhone OS 3.1.2, you will need to download and install the new iPhone SDK to continue your iPhone app development.
A version of iPhone SDK 3.1.2 is also available to developers who are running Mac OS X Snow Leopard. Please ensure you select the appropriate SDK based on your iPhone app development environment.
iPhone SDK 3.1.2 is now available