OK, that's kind of what I thought. The idea came to me when I saw a CD I left on the front seat of my SUV (bad disk, was going to throw it away later, not worried about heat death!) and it flashed in the sun. Thanks for the replies, especially Susan -- it's the clearest of many good reasons not to.

Although the bus-card CD-Rs would probably make a good way to back
up important documents to be kept in a PSK. Just not as a good signal mirror. -Gary

EDIT: I followed Chris's advice and read DR's page on signal mirrors. And wouldn't you know, right there in the equivalent of bold type: why a CD-R isn't a great idea. My apologies, I read the forums everyday, and got caught forgetting there is more to this website than just the forums.