I just received the following e-mail from Apple:
iPhone SDK 3.1 beta and iPhone OS 3.1 beta are now posted to the iPhone Dev Center. These versions are for development and testing only and should be installed on devices dedicated to iPhone OS 3.1 beta software development. Please read the iPhone OS Pre-Install Advisory and the iPhone SDK 3.1 beta release notes before downloading and installing.
Use the Apple Bug Reporter if you discover compatibility or functionality issues while testing with the iPhone OS 3.1 beta release.
iPhone SDK 3.1 beta and iPhone OS 3.1 beta is now available to developers. If you're a developer head on over to the iPhone Developer center and get the latest iPhone 3.1 beta download!
Update: ultrasn0w users, do not update and read my iPhone OS 3.1 updated to baseband 05.08.01 post. Also read my What's new in iPhone OS 3.1 beta post to find out about the new features.