Apple announced the availability of iOS 4.0 on iPhone 3Gs devices only, pretty much requiring you to buy a new iPhone 4 if you have an older iPhone 3G.
Well, the Redsn0w jailbreak tool is here that will allow you to jailbreak iPhone 3G AND install iOS 4.0.
redsn0w beta has been updated to hacktivate iOS 4.0 for iPhone3G (in addition to jailbreaking the iPhone3G and iPod Touch 2G). The download links for redsn0w 0.9.5b5-3 are:
Of course, this is all still *experimental* so I would advice to wait couple weeks until stable hacks