Apple has just rolled out iOS 4.1 Beta release for developers. If you have a developer account, you can download it from the Developer Center. All those who wish to retain iOS 4 jailbreak should stay away from the iOS 4.1 update. More information should be available soon as to if iOS 4.1 was safe for jailbreaking or not.
To download iOS 4.1 Beta, head over to Apple Developer Center, log in and you should see the update.