Chronic Dev team has released Greenpois0n RC5. Without SHSH blobs, GreenPois0n RC5 can untether jailbreak iOS 4.2.1 on iPhone 3GS, iPhone 4, Apple TV2, iPod Touch 3G and iPod Touch 4G. The bad thing on this jailbreaking tool is that currently it is available only for Mac OS X. But good thing is that Linux and Windows version of this tool would be released very soon 🙂 .
Download Greenpois0n RC5 to Untethered iOS 4.2.1 Jailbreak
Some reports are claiming about Greenpois0n RC5 destroying All USB ported on Mac OS X.