As far as I know, it's always been a requirement, though after entering once, you can install more apps without entering a password for a temporary period. It's a good idea, too, since your AppleID is tied to your debit card or credit card, so if someone got a hold of your unlocked iDevice, if the AppleID password wasn't required, they could install as many apps as your debit card or credit card will allow. That's no good. Getting a refund for an AppStore App is near impossible.