I just had my very first earthquake (not just in Japan; the first earthquake of my entire life). I was at home, in my room, when I noticed the room shake. It was very weak, and lasted only a few seconds. In fact, it was so short that I wasn't sure if I'd imagined it or not.
But according to the Japanese Meteorological Agency, there was an earthquake at 21:01 JST, May 8th 2007. It had a magnitude of 4.4 on the Richter schale, but the epicentre was quite a bit away from here, which explains why I barely noticed it (and hardly anyone else here noticed it at all). The JMA uses a scale called the JMA Seismic Intensity Scale. By that scale, the quake was intensity 3 at the centre. According to their breakdown by location, the intensity in this area (Setagaya-ku) was only 1.
The JMA has information about the most recent quakes here. The information about this quake will be on there for about a week (it appears there has already been a more recent quake so this one is off the front page; you can find this one by clicking on of the buttons at the top and then selecting it from the list; that approach should work for a week, then it's gone).
UPDATE: MSNBC writes about the quake.