I was wondering the same thing, especially in regards to the AP test: my high school has a fund set for that sort of thing and they'll pay for everything, so I might as well take the thing, and if I'm going to sacrifice a few hours of my life, I might as well do well.
There probably won't be any mainstream study guides for AP Japanese (to the best of my knowledge, Princeton Review doesn't have anything for German or Latin, although there might not be an AP test at all for the former) as the test was first held just just two years ago in 2006: it might be too early for this sort of thing.
