iH8Sn0w, the dev behind Sn0wbreeze, has managed to jailbreak iOS 4.2 beta on iPhone 3GS (old bootrom) with Sn0wBreeze. He tweeted about successful iOS 4.2 jailbreak on iPhone 3GS with old bootrom and that Cydia, Icy or Rock are not currently working on jailbroken iOS 4.2 beta . This is what he tweeted:
So I just jailbroke by 3GS (Old bootrom) on 4.2 and Cydia/Icy/Rock wont work… Icy just wont update but launches. I haz ssh though.
Please note that all these Sn0wbreez jailbreaks for iPhone 3GS works for old bootrom only, for rest of you guys Greenpois0n is the only hope for now which is based on a new bootrom exploit and will jailbreak iPhone 4, 3GS, 3G, iPod Touch 4G, 3G, 2G and iPad.
Basically, the updated operating system has been jailbroken using both PwnageTool for Mac, and with Sn0wbreeze for Windows machines. Incredible how fast these guys work!
We would fully expect Apple to make this exploit begone in the final version.