Ha Ha! Not even a month ago, the same thing happened to my car. The difference is i could see the mirror wobbling in the mind, and I peeled it off before it got a chance to fly off. The problem at this time of year is that water gets behind it, and freezes. After many cycles of that, it loosens up.

Anyway, i replaced mine with Gorilla glue from Home Depot. It has held on well so far. 4 pieces of advice:

1. I made the mistake of giving the back surface a light pass of 220-grit sand paper to give the glue a little extra surface to bite. HUUUUUGE mistake. I didn't realize it until after i glued it back up: you can see each and every scratch on the front of the mirror. What a bonehead move on m part.

2. Don't try to pick the old black adhesive off the mirror mount. It's spongy, and somehow integral in the mirror's heating function.

3. If you go the Gorilla Glue route, remember that it expands as it dries. Don't freak out when you see it oozing a little bit around the edges of the mirror. It trims off easily with a razor blade.

4. I found that two of these clamps worked great to hold the mirror in place while the glue set. http://www.harborfreight.com/3-inch-spring-clamp-39530.html I actually used a plastic version of this clamp. It's not nearly as forcefu (i.e. won't crack your new mirror) Most good glues have a long cure time, so just set it up in your garage int he evening, and it will be good to go by the morning.