The Japanese Kei Car pool on Flickr came through.
As it turns out, the strange vehicle I saw this morning was a Nissan S-Cargo (snail pun probably intended). According to the Wikipedia article it was a small van produced in Japan from 1989-1992. S-Cargo vans(?) were exported for use in other countries, and are coveted as business advertisement vehicles for their rarity and abundance of ad space.
I also ran across another resource with tech specs for the Nissan S-Cargo.