The Luminous-Landscape has an open letter to the camera manufacturers. Interesting and fascinating read. Basically, depending on the individual camera, you could lose as much as a full stop of light by the time the light hits the sensor, according to the DxO Lab's findings. Most APS-C sized sensors would lose anywhere from 0.75 EV to 1 EV. So, the actual amount of light from an f1.2 lens on the Canon 450D that the sensor receives is around f1.8. Of course, you will still get the exposure value of f1.2, because the manufacturer raised the ISO to compensate. This means when you set the ISO to 100 when at f1.2 on the 450D, it's actually f1.2 at ISO 200.
Should you care? With today's high performance sensors, most people probably won't notice, or even care even if they know what's going on behind the sensor. But that's kind of cheating, me thinks.
My take on this is this: Manufacturers should publish the behaviour of the cameras they make. If the sensor raises ISO to compensate for the loss of light, they should mention that fact. But, I also don't think it's a big deal, as long as you understand the image characteristics of a particular camera. After all, the final image is what counts.