Great UI design and concept (points is a great incentive here). It looks very approachable and something I could actually use. You can probably capitalize on recommending learning materials etc (like what you did with "improve your SAT by 200 with kaplan"). Overall really awesome product.
Setup a bare git repository in your DropBox folder and clone from it into your development folder. The DropBox folder will act as your "remote" origin. Now you can push and pull changes to it. It will physically get backed up when DropBox notices file changes.
Sexy Graphics... but I would revamp some of the interface arrangement. You should follow a more standard style whereby the features can more easily be read or that the teaser page in general can be ready quickly.