iPhone Dev Team announced just a few minutes back that they were working hard to release tools to jailbreak and unlock iOS 4 and had warned users who relied on software method to unlock iPhone to avoid upgrading to iOS 4 until they have tested their tools with all relevant devices.
But they have just released the latest version of Ultrasn0w, their software solution to unlock all basebands since iPhone OS 3.0 including baseband 05.13.04 that comes bundled with iOS 4.