I have been told by colleagues who visit the US often that if you've been through US immigration with the same passport and had your fingerprints taken before, they only take the first fingerprinting step (the 4 fingers on the right hand). If this matches the previous record, they are happy that the rest of your fingerprints and photo are already on record so do not ask for them (or the system does this automatically). Someone please confirm this though as I haven't experienced this myself having only been to the US once since the fingerprinting came in and, obviously, had the full set taken.