So I just found out that Ubisoft released this sometime last week:
I've been giving it a shot, and if you own a DS and don't know any Japanese (but want to learn), it seems like a decent enough option to try. It teaches kana, basic grammatical constructs, vocabulary and (I think) kanji, through a combination of lessons and exercises.
However, if you already know some Japanese and basically want advanced lessons (which is what I was looking for), you may want to look for other options, because there isn't any way to skip the first set of basic lessons. I'm about 35 lessons in and it hasn't even started introducing kanji yet (whereas in my university course the professor introduced us to our first kanji after just a few classes).
Anyone else want to weigh in on this sort of stuff?