Good Japanese in the Financial District??
What kind of Japanese? There's cheap lunch sushi at Kamakura on Battery or, much better but farther, Mikaku on Grant. Kyo-ya is expensive and upscale, and Ozumo is hip and creative. Cafe Tomo on 1st has some Japanese-inspired items, though I don't like it as much as others do.
I work down in the same area, and have been looking for the past year and a half. I would agree with David's comments, and add that my favorite these days is Delica in the Ferry Building. It is very clean/healthy, yet tastes quite authentic (I lived in Tokyo for a couple of years and ate a LOT of Japanese food there ;-)
I'd recommend their various bento boxes, the curries, and the noodles, in roughly that order. The tonkatsu is decent, and all of it is uniformly high quality.
Ozumo is a nice business lunch sort of place, but more expensive, and often too noisy for me.