Hey all,
Here's how I solved this problem once and for all a few weeks ago...
There is one sensor for the front, and one for the rear (front driver, rear passenger on US market cars, I believe). You can replace the sensors, as many folks have mentioned, and that should get rid of the light. However, you will need to replace these sensors every time you change pads if you let the light come on (not a huge deal, but one more thing to do). I also ran into problems with mine popping out when running through water at a decent rate of speed, and the light coming on unnecessarily, so I eliminated them. All you need to do is cut off the "sensor" at the end of the wire, and short the two remaining ends together. Done. No more warning light, ever. I know a couple of other guys who have done this as well, with no ill effect so far. Just pay attention to your brake pads, since you will no longer be warned when they are getting low.
Paul