Jailbreak released for all iOS4 devices!
Comex has released his jailbreak for all iOS4 devices, including the latest iPhone 4. The official site is http://www.jailbreakme.com.
Once you have jailbroken your iOS4 iDevice with ‘JailbreakMe’, you will be able to unlock it using UltraSn0w (this would NOT apply to factory unlocked devices by default). The UltraSn0w unlock would not work for iPhone 4 right now.