Some history... back in the early 1990s, Motorola decided that there was a huge market for phones which could be used anywhere. A traveling businessman wouldn't have to get one phone for when he was in New York, one for Los Angeles, one for London, and one for the midwest. Instead, he could just carry one satellite phone and be done with it. Motorola would charge around $3 a minute to use the phone and businesses would be more than happy to pay. They planned a constellation of 77 satellites to continuously cover the entire globe and named it Iridium since Ir has 77 protons. Over the course of planning though, this got cut down to 72... name didn't get changed to hafnium though.
Unfortunately, the launches didn't happen until 1998, and by then national and international cell networks had spread and basically made it ridiculous to pay $3/min for something you could get much cheaper. The quality was also far worse than regular cell phones. So Motorola was loosing money hand over fist and came up with a plan to de-orbit the satellites since they weren't making enough to cover upkeep on the ground communication stations. At the very last minute, a private capitol group bought the whole system for $25 million (it cost about $6 billion to build).
The cool thing (or uncool thing if you don't like light pollution) is that the solar panels on each one of the satellites are very reflective to prevent overheating. Basically, each satellite has three plane mirrors on it. In order to work, the spacecraft have to keep a very carefully adjusted geometry relative to the Earth and the orbits are well known. Knowing this means that it's not too hard to figure out exactly when a specular reflection of the Sun will hit a given area on the Earth. When you do get one of these reflections, it's bright... typically around magnitude -8. For comparison, the "bright" stars you see in any random area of the sky are magnitude 1 or so. The magnitude system is logarithmic with a difference of five magnitudes corresponding to a factor of 100 in brightness. This means that the reflection from the Iridium satellite, a "Iridium Flare", is something like times brighter than most of the bright stars in the sky.
Since the geometry is well understood, there are several websites that predict when the flares will happen. I've been using Heavens Above where you can input your latitude and longitude and get flare information as well as information about other satellites, shuttle passes, and the ISS. This particular flare hit a magnitude of -7 at 22:40:14 on May 4, 2010, and came from the Iridium 40 satellite. My location in Walla Walla was about 6 km from the actual track of the reflection, but that apparently didn't dim it too much. The peak of the light (middle of the streak) was at an altitude of 24 degrees and a north azimuth of 45 degrees.