The first jailbreak for the iPhone 4 and iOS 4.0 has been released, and it's available now via mobile Safari.
JailBreakMe 2.0 is a tool for jailbreaking not only the iPhone 4, but any iPhone running iOS 4.0.1 or below and any iPad running iOS 3.2.1 or below. What's especially unique about this jailbreak is that it can be implemented simply by visiting the JailBreakMe website, making the process far less complicated than most hacks.
As always, we want to remind you that jailbreaking your iPhone will void its warranty, even if it is now legal. Initial reports seem to indicate that JailbreakMe does indeed work properly, if you can get through the website's currently slammed servers. However, there are reports that it can break FaceTime and MMS (though the issue can be fixed).