The card game is accessed by the "Combat" button, which brings up a list of friends to battle (in a Mafia Wars style). You select the friend you wish to battle, and the card game will begin. Unfortunately, this isn't true PvP because you aren't actually playing with your friend. Your friend is controlled by the game's AI and you have to pick the best moves to defeat them. This isn't a simple game though, and Playdom have recognized this by making a guide to the new questing card game. You do have to use tactics and pick your damaging and healing abilities correctly in order to beat your friend in the card game.
It's not flawless, but this level of depth in a social game goes a long way to combat the negativity that surrounds these type of games - critics are constantly insulting them for being nothing but clicks, no depth, and no skill. The question is, do players want this? In the past month, Treetopia has been losing some momentum but still has over 2.3 million monthly players. Time will tell.