Just checked the one on my 03 Camry. It is NOT polarized. Used a set of polarized sunglasses to confirm this on your car, if you want. Hold the glasses up near the moonroof, and look through both. Then rotate the glasses clockwise (or counter if you prefer...) through 360 degrees. If the moonroof is polarized, when the lenses are 90 degrees out of phase, that will effectively block almost all light, and the lens will appear to go almost black. If you're unfamiliar with what I'm talking about, try comparing two sets of known polarized glasses against one another in the same way. The out-of-phase effect is immediate and obvious. You can also try this with Toyota's curiously polarized radio and odometer displays (they are in Gen-5s; I don't know about earlier cars). While wearing polarized glasses, slowly tip your head over to the side. When the displays suddenly go black, that's when you're out of phase.
All this is really academic, however, as to your primary concern. Polarization is not specifically going to do much about the really bad stuff from the sun, UV radiation. I suppose the UV that comes at you in the plane filtered by the polarization filter would be cut out, but all the rest of it will still come right through. Look for UV filtering efficiency.
EDIT: My last physics was years ago, and I'm having a nagging feeling that the "phase" terminology is incorrect. For those who care, I'm talking about the angle of the plane in which the light waves are traveling. I hate saying it, but, "you know what I mean. . ."
Thanks for the feedback. Maybe someone has a more technical answer for those of us who distantly removed from our physics class. How about tinting the glass with a UV protected film? I never even had a class on window tinting.