This is a question I've asked myself a couple of times. It appears to be one of those 'things' that was lost during the OS 9 > OS X transition. Most of the missing features have eventually made it back (i.e. scheduled startup/shutdown), but Voiceprint is one of those things that has never been reinstated. I'm not saying it was particularly brilliant, I just remember Apple being pretty proud of it as the time and it's odd that such a nifty feature hasn't been developed.