Apple today, released the 3.1 beta for the iPhone firmware and SDK; both the SDK and the firmware are available for 
testing and development to paid members of the iPhone developer program. Although this is pre-release software and details should remain under NDA, some details will leak out soon.
Some wonder why Apple have released the 3.1 Beta so close to the release of 3.0 to the public. Some think that it’s not just bug fixes, as Apple would probably release it as 3.0.1 if so. Apple may have worked out the exploit in which the Dev-Team use to jailbrake devices and patched it in this new release, so upgrading is to be taken at your own risk and may stop you from jailbraking until the Dev-Team work it out.
Sam's Blog 