The second time the Mac it will be completely ignored, ie 3GS on the cable is not recognized.
BUT: The same cable under Win XP or right on the charger is working properly !! So in this case, there is then only the loading function or under XP I use no iTunes, so do not tell you sync possible, but the Mac after the 2nd time all over: 3GS is ignored / not recognized.
Without Sync it is of little use to me, therefore, 3 star deduction.