We had just published our step-by-step guide to jailbreak iOS 4 using Redsn0w for iPhone 3G for Mac users. This guide is for Windows users.
Redsn0w 0.9.5 To Jailbreak iOS 4.0 GM Released, Enables Multitasking, Custom Homescreen Wallpaper For iPhone 3G
Today they have released an updated version of redSn0w (version: 0.8) which can also jailbreak iPhone 3GS.
Dev Team has also released an updated version of UltraSn0w (version: 0.8) which can unlock Apple's third generation iPhone.
So with this, the Dev team has released updated version of their tools which support all iPhones and iPod Touches running iPhone OS 3.0.