Apparently, an ex-Apple employee has managed to successfully jailbreak iPhone 3GS on iPhone OS 4 Beta. The redsn0w 0.9.5 jailbreak tool for iPhone OS 4 currently works on iPhone 3G only. The support for other iDevices will naturally follow once Apple releases iPhone OS 4 at WWDC. But KaatjeNL’s jailbreak method works on iPhone 3GS too. And the best thing about it is that it doesn't upgrade your baseband, which means you can enjoy iPhone OS 4, fully jailbroken on the older baseband with an unlock using ultrasn0w.
Well it now looks like Geohot is not going to release his jailbreaking bits for the iPhone and iPad anytime soon. According to some of his recent tweets, his tools are not ready for primetime use yet. What this probably means now is that he is not going to release blackra1n for untethered iPhone 3.1.3 jailbreak (as previously thought), iPhone OS 4 and iPad before the next-iPhone is out this summer.