If you’re trying out the new iPhone 4.0 firmware and interested in jailbreaking it, then you can safely use Redsn0w 0.9.5 to jailbreak iPhone 4.0 This is not the final version, you have to use it on your own risk.

This version is currently a Mac only and is meant to be used by developers to test the jailbreaking on this firmware on all possible grounds. The Redsn0w 0.9.5 is for iPhone 3G only running on iPhone OS 4.0 beta!

You can download Redsn0w from here and see its usage instructions from here.