I dunno... I wouldn't want to rely on it as my only way to start a fire, but:

• I did polish the bottom to mirror smooth using toothpaste, my thumb and 40 minutes of my life I don't get back.
• I did manage to get dark newspaper to smoke nicely in bright sunlight using the mirrored end.
• I did manage to get a coal in a piece of cotton cloth rubbed with charcoal, using the mirrored end.
• I started quite a nice fire by transferring the coal to a handful of dried grass and blowing, gently.

It works, but in a fairly narrow range of conditions. It's neat to try at least once, if only just so you can say "Hey... watch this." (I have my polished Mt. Dew can on my desk right now. <img src="/images/graemlins/smile.gif" alt="" /> )