Proceed with caution because this can and will brick your iPhone but most reports indicate smooth sailing.
1. Backup up your iOS device with iTunes.
2. Ensure your iOS device has been updated to iOS 4.0.1. or 3.2.1 for the iPad.
3. Enter http://www.jailbreakme.com from your iOS device’s Safari browser.
4. Follow the on-screen directions and try not to freak out.
5. Once finished you will be greeted with a notification that says, “Cydia has been added to the homescreen.”
6. Your iPhone/iPod Touch/iPad is now jailbroken. Enjoy.