It looks like American Girl's rewards program went live earlier today. You can register online using your americangirl.com user ID or you can create a new account on the website. When you register the site shows your current reward tier, points to next tier, and tier expiration date. I'm not certain but it seems like you may have to maintain a certain purchase level to remain on the higher tiers.
I'm looking forward to seeing how the rewards program will develop over time.