Considering that players like Orestes Destrade, Boomer Wells, Alex Cabrera and Tuffy Rhodes (all marginal major leaguers/AAA stars) have been dominant players in NPB over the last 25 years, I'd say that with the exception of a handful of stars, NPB is at roughly a AAA level of talent. If the 2009 Yomiuri Giants, winner of last year's Japan Series, were to be dropped into the American major leagues, they'd almost certainly lose 100+ games. Their defense would be good and their pitching passable, but their would have trouble against good AL or NL pitching and comparatively large ball fields.
Examples abound. You know Kaz Matsui as a journeyman 2B with middling offense and a good glove. That's in the American MLB; before that, he may have been the best shortstop in NPB history. Daisuke Matsuzaka in Japan led his league in wins three times, in Ks four times, in ERA twice and pulled in a Sawamura Award (their Cy Young Award); over here he's been just a middle-of-the-road starter. Ichiro is probably the only Japanese player to come to the US and be just as good as he was at home (even Hideki Matsui had better NPB than MLB numbers).
That being said, I'm still looking forward to seeing Yu Darvish in an SF Giants uni someday. But he's the exception -- and even so, he's already won two Pacific League MVPs and he's only 23, nowhere near his peak years. It says a lot about the competition he's facing.
Wikipedia, 2010 Baseball Prospectus, The Baseball Guru website