One of the more popular Userland Jailbreak (Not Firmware Based) that is Untethered (Reboots don’t require you to Jailbreak again) software called “Spirit” has been updated to work with iTunes 9.2 which was released to support the new iPhone 4 and iOS4 firmware.
This new version doesn’t add support fr iOS4 or the iPhone 4 at this time, you can download and read more about the progress of Spirit at their website. Oh and Spirit works on Windows/Mac/Linux.
