Apple offered us a preview of iPhone OS 4.0 just over a week ago and already a jailbreak is ready for developers to download. The jailbreak app is called redsn0w 0.9.5 beta and is the real deal, unlike some of the other claims made over the past few days.
This beta release will now allow devs to prepare for the summer release of Apple iPhone OS 4.0 on the iPhone 3G and not the iPad. Although Engadget does say that it is early days so we could see a iPad jailbreak for the new OS soon.
If you rely on a carrier unlock then you had best stay clear of this jailbreak, this beta download will only work if you have stock 4.0beta1 IPSW from Apple. If you already have IPSW installed, then you would have already lost the carrier unlock. For more details on this visit the iPhone Dev Team
Please remember, this download is only for Devs