[Buzz]

Interesting question. The foil may have a slight insulating effect if there is air trapped between the can and the outer layer, but foil is a pretty good conductor by itself so in direct contact with the can it should have little effect. Now, if the can is potentially exposed to a source of radiant heat - sitting out in direct sunlight for example - then the shiny side of the foil could act to reflect some of the radiation and would heat up at a much slower rate than the surface of the can if it is exposed.
I did a search on the topic and the following post came up. Some of what the guy wrote is plausible when talking about grilling and cooking because the foil can indeed reflect radiant heat. When he tries to explain the results of his soda can experiments, however, he talks about the foil reflecting "cold". Cold is not a radiated form of energy - it is simply the absence of heat and can only be "lost" through conduction or "conserved" by insulation. It can not be "reflected"!
( I put those words in quotes because you can't actually lose or conserve "cold" you can only speed or retard heating)
Still, if his story is true - some interesting results:

dalepetrie said:
I see these answers which say either it doesn't matter, or put the shiny side towards the food but it doesn't matter much. I can attest that it DOES make a huge difference. The shiny side reflects, the dull side absorbs. Now yes, on a grill, the heat is so intense that likely what's going to happen is that whatever side faces the flame is going to get blackened, and the heat will go through just as quickly. But when I was in high school lo a couple decades or more ago, I would pack a can of soda for with my lunch. But alas, I had no refrigerator, there was no way to get ice at the school cafeteria, and I couldn't stand warm soda. So, I began experimenting with tinfoil. Here's what I found:

Taking a refrigerated soda - warm by the time I got to lunch.
Taking a refrigerated soda wrapped in foil with the dull side facing the can - coolish by the time I got to lunch.
Taking a refrigerated soda wrapped in foil with the shiny side facing the can - mostly cold, but not as cold as I liked by the time I got to lunch.
Wrapping a soda can in tinfoil, shiny side towards the can - can would freeze completely within an hour, soda still frozen if I didn't take the foil off.

I found two ways to get the soda as cold as it would be if I took it out of the refrigerator. Either take a refrigerated can, put it in the freezer for about 15-20 minutes w/ no foil and wrap it with foil shiny side in, OR, at 10pm, I would take a can, wrap it in foil with the shiny side out, place it in the freezer, then at 7:30 the next morning, I could put it in my bag and at lunch time it was perfect. The shiny side reflected enough cold so that it wouldn't freeze in 9 1/2 hours in the freezer (a regular can put in the freezer with no tinfoil will usually explode within an hour and a half), yet, it would allow enough cold to get in, and keep enough cold in over the next 4 1/2 hours to make it the perfect temp.

So, it actually DOES make a significant difference in some cases.

Another example, baking a potato. I have cooked potatoes in the oven without foil, they get done in about an hour. If I wrap them in foil shiny side in, they are done in 45 minutes. If I wrap them in foil shiny side out, they take an hour and a half. The only other difference is the skin gets soft if cooked in foil and gets crispy if cooked w/o foil.

Bottom line, if you're grilling on the bottom rack where the foil is sure to get sooty, then it probably isn't going to make a HUGE difference, but if you're wrapping something in foil and putting it on the top rack, then it will take you twice as long to cook if you put the shiny side towards the heat.