Ultrasnow is now available for download to unlock iOS4 on iphone 3G and 3GS handsets. iPhone OS 4, set to ship later this summer, has already been jailbroken. The iPhone Dev Team has released a beta version of the ultrasn0w jailbreaking tool.
But they caution users that it’s definitely not for the casual jailbreaker and can easily break your iphone. At this moment , it only supports jailbreaking iPhone OS 4.0 beta 1 on iPhone 3G device and the jailbreaking tool only runs on Mac OS X operating System. So those on Windows PC cannot run it to jailbreak iphone 4.0 at the moment.