I personally think it's the wrong approach to learning a language. Plain memorization is not how languages are learned. There is a lot of context and structure that must be learned that isn't apparent in the lessons (just listen to someone with a foreign native language speak English. They will construct sentences with English words but will do so incorrectly. They are putting them together how they would if they were using their own language's words).

I certainly think this is a problem ripe for hacking, but I think the application won't stick. What I've seen so far is a decent supplement to a traditional course for those who want to broaden their vocabulary repository, but not (IMO) a complete way to learn a language.

Another note: What about for people who already know a portion of a certain language? Maybe you could implement a test that checks where they are and provides lessons accordingly.