Jan 10
From Apple Developer News:
Beginning January 9, app screenshots will be locked in iTunes Connect once your app has been approved. New screenshots may be uploaded when you submit a binary for an update to an existing app or a new app.
This is in response to the tactic employed by some scam apps whereby they switched the screenshots after the app had been approved to look like a popular app with the hope of tricking customers into downloading the wrong app. See this blog post from Panic for an example.
Sigh.