I am going to wait a bit to see how well this jailbreak works on the iPhone 3G, running 4.0.1; this is not touching my iPhone 4. Until then, here is the direct link to JailbreakMe.
The userland jailbreak for iPhone 4, iPhone 3GS, 3G on iOS 4 / 4.0.1, iPod touch on iOS 4, and iPad on iOS 3.2.1 is out !!. It is a browser based jailbreak, all you have to do is to browse to http://jailbreakme.com from your iOS device to jailbreak it. Servers are probably thrashed right now. Keep trying until you successfully load the page. We will be back with quick how-to guides soon. Stay tuned for that !!
Joshua Topolsky, Twitter
iOS4 jailbreak is out — word of warning, it can brick your phone (it bricked ours!)
Turns out a lot of users are experiencing serious issues with JailbreakMe, and for now I must advise against it.