You'll get Arcade Credits for getting high scores, and for keeping them high too.. you're not only awarded credits when you gain a high score, but you'll gain a certain ammount of credits per day for keeping youself in the highest of positions too.
Challenges are a new feature to the arcade too, allowing you to use these Credits to place wagers against other arcade players, quite simply choose your challenger, pick a game, and play it.. the other challenger will receive a request, and if they accept and play the game, the player with the highest ending score will win, and take home the credits.
You can also place friendly challenges against other players by setting the credits to zero, which will simply show up on the front page of the site, and on your player profile page