p0sixninja, an important figure behind Greenpois0n posted on his twitter account that Greenpois0n to jailbreak iOS 4.2.1 is coming within next few days. Greenpois0n is based on Geohot’s limera1n exploit, an hardware-level exploit, and Apple can’t patch it up until new iPhones and iPads are released with improved hardware.
Greenpois0n for iOS 4.2 will be tethered and semi-tethered which means you’ll have to connect your idevice to PC to get the jailbroken state on every boot.
GreenPois0n should be updated to support 4.2.1 (tethered or semitethered) within a few days
However the old devices like iPhone 3G (old bootrom), iPhone 3G and iPod Touch 2G can be jailbroken untethered using Redsnow.You might also like to read these posts:
Is there a specific date for release?
No ETA yet.