To go along with iPhone OS 3.1.2 (and the iPod touch equivalent) released earlier today, Apple has sent an email to registered developers informing them that:

Phone SDK 3.1.2 is now available on the iPhone Dev Center. If you have updated your development devices to iPhone OS 3.1.2, you will need to download and install the new iPhone SDK to continue your 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 development environment.