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Message: <blockquote data-quote="Jaygray" data-source="post: 1093826" data-attributes="member: 50033">Unless you were previously jailbroken at 4.0 and had Cydia installed on your phone, you can not downgrade a 3GS from 4.0.1 to 4.0.The 3GS is designed to check with Apple and verify that you are installing the most recent firmware.  The 3G does not have this ability and can easily be downgraded, but the 3GS can not.That being said, there is no reason you can't simply jailbreak at 4.0.1.  <a href="http://www.jailbreakme.com" target="_blank">JailbreakMe</a>After jailbreaking, install Ultrasnow via Cydia and you should be unlocked.</blockquote>