Greetings!
Badges, cards, profile backgrounds and emoticons are already done, but cards will not start to drop until the game has reached a confidence metric that makes it clear it's actually being bought and played by genuine users. Once a game reaches that metric, cards will drop to all users, including all the users who've played the game prior to that point. So going forward, even if you play a game before it has Trading Cards, you'll receive cards for your playtime!