Hi jsgrwn - i have seen this mentioned before (i think it was in some marijuana growing bible or book or something) but it isn't really true about mirrors absorbing light. I mean yes, everything absorbs light to some degree (except of course - light) but mirrors are one of the more perfect reflectors of light. The problem with using mirrors for growing is they can create hot spots if the angle of the mirror and bulb line up just right. If you want to see this for yourself, take a bright flashlight and shine it into a mirror and move the flashlight around until you are looking at the bulb through the mirror. Very bright isn't it? i wouldn't call that absorbing the light.
i use them to reflect the light up so the under branches get plenty of light as well so not only is the top of the plant getting direct light so does the bottom.