iOS 4.1 is now available for download, and I'm pretty sure some of you will "accidentally" update to 4.1 and will want to downgrade back to iOS 4.0.1, which is the latest jailbreakable and unlockable iOS (at least at the time I'm writing this).
This simple guide and tutorial will show you how to downgrade your iPhone iOS 4.1 to iOS 4.0.2, or 4.0.1, or iOS 4, or any earlier version. The steps are the same no matter what iOS you want to downgrade to.