We have a litter of Aussies right now (our 2nd litter) and they just turned 4 weeks. In some of them I could see "personality" as early as 2-3 weeks but now they are all really showing their spunk or their easy laidback attitudes. I'm on the watch with two of the girls though because the buyer wants the "most tenatious one" and I'm going back and forth between them trying to decide. One day one will be so sweet and the other real bossy and outspoken and then they switch.
But there is definitely already a "shy one" and a "super sweet lovey boy". Seeing them every day, all day is a lot easier than just having a little bit of time with them.