These are good points about checking the details carefully. I got really burned using an AT&T card in China a few years back so I'm REALLY careful.

The rec to use masterbell has turned out as advertised: 1 minute billing, no setup fee, no monthly fee, 6 cents/min from a real Tokyo local call.

Slightly off topic, I also rented a cell phone for my daughter to use in Japan, from a company I've used before, and based on a flyertalk rec: Rentaphone Japan. These guys are by far the cheapest, and everything just works.