iPhone Dev-Team released an iPad 3G jailbreak on their site codenamed “Spirit,” provides an untethered jaibreak on those newer devices which used to require a computer nearby to finish the boot process. Spirit is able to do this because it doesn’t actually kick in until after the kernel is running. “Spirit” and was first demonstrated working on an iPad by @MuscleNerd within 24 hours of the iPad’s release on April 3. Userland jailbreaks are more troublesome for Apple since they expose security weaknesses that exist even for non-jailbroken owners. As such, Apple is likely to close them soon after they’re made public. One recent example of this’s the SMS vulnerability exposed at Blackhat last summer. Apple released new firmware to close that hole within a day. The Spirit jailbreak is most useful for newer devices: iPhone 3GS, iPod Touch 3G, and the iPads. Unfortunately those devices are the same ones that Apple can prevent you from downgrading unless you’ve got a backup of your personalized SHSH blobs. Unless you’ve backed up your SHSH blobs for vulnerable firmware versions, you’ll lose the ability to use the current Spirit jailbreak if you accidentally upgrade.
More info: iPhone Dev-Team blog