It worked because your phone has old bootrom and has saved shsh with cydia..otherwise you cannot downgrade and unlock yet...we have to wait for ios 4.01 unlock which should come out soon..
yes it is true.may be u try the easiest way to downgrade os 4 to 3.1.3 3G n 3Gs.if u end with error 3194.keep calm and wait for ios4 jailbreak.dont waste u r time.