Nice! This is the first time I've seen the dependency graph implemented in e-learning! Actually I started a programming site in spanish a while ago with the same idea (there is a dependency graph underlying the content). If you are curious: http://www.minidosis.org.
Doesn't the Khan Academy use dependency graphs? I had some fu Ngoing through highschool calculus on their site. Raking in the achievements and unlocking next lessons.
