The iPhone 4 ships standard with some of the features that led many people to jailbreak earlier iPhone models. You can change the home screen wallpaper. You can run some activities, like streaming music or VoIP phone calls in the background while using other apps. But if you want full control of your operating system, including the ability to install third party apps that aren’t available in the App Store, you’re going to want to at least consider jailbreaking.
And now just a few days after the hardware became widely available, at least a few people have figured out how to gain root access to all of their phones’ settings.
It will probably be a little while before we see the latest jailbreak tools released to the public. They currently include some proprietary code from Apple. And the current jailbreak method won’t give you the ability to use your iPhone 4 with a wireless carrier other than AT&T, because the software controlling that functionality has been completely rewritten in the latest iPhone. But I suspect we’ll eventually see hackers figure out a way around that restriction.