Apple today released the iPhone SDK 3.1 beta, which works with the upcoming 3.1 FW release. A list of the few fixes are available for developers, but its nothing major - just the 3.1 support really.
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.