The Canadian iPhone hacker iH8sn0w have confirmed that iPhone OS 4.0 Jailbreak is very much possible. Although iPhone OS 4.0 support multitasking (only for iPhone 3GS and 3rd-Gen iPod Touch) but like other firmwares it won't allow you to install non-App Store apps. iH8sn0w confirmed this via his Twitter account that he have got iPhone OS 4.0 decryption keys with which one can easily decrypt the whole firmware and Jailbreak it. However he claims that he do not have an exploit for iPhone OS 4.0 Jailbreak.
iH8snow is an infamous iPhone hacker behind various iPhone jailbreaking and unlocking tools like sn0wbreeze, f0recast and BlackBreeze.
iPhone OS 4.0 was released by Apple yesterday at iPhone 4.0 event which held at 1-Infinite loop campus in Cupertino. The main highlights of iPhone OS 4.0 include Multitasking, Game Center, Folder Feature and support for 5x Digital Zoom.
Update 2: iH8sn0w Jailbreaks iPhone 4.0 OS As WellContact Us for News Tips, Corrections and Feedback