JaguarFan, although I get replies from P whenever I sent them updates, I do not think it is of any significance. You must understand that there are several people surfing through incoming mails, some reply, some dont- no biggie.
And I hardly think getting a notification on your update is a clue to how much Princeton 'values' you or if its an insight into your admissions chances, I think P has much bigger things to tackle atm than decide whose emails are worthy replying to, and whose are not