iPhone 4 got successfully jailbroken! Probably, the ‘userland’ jailbreak vulnerability (original method by @comex) was used to get root access of the device by @planetbeing and @chpwn. You should know that it’s just a preliminary jailbreak. That means right now it is low level and easy for Apple to patch.

The iPhone dev team member MuscleNerd has posted a screenshot of the jailbroken iPhone 4 running Cydia.