Well Dirtgirl, I just couldn't stand seeing your post not get answered anymore! I didn't respond earlier, because I've never seen foil in my carolina wrens' nests. It's very interesting though! Those birds find the strangest/funniest places to build their nests! One year, we had our table saw on our big, covered front porch, with a tarp over it (yes, we're pretty fancy here), and those little guys built a nest right under the tarp! Fortunately, we didn't need to use the saw for awhile.
I have tarps for an awning in front of my chicken coop, that I keep drawn up in summer, and pulled down in winter, to keep the wind out. Well, there's a nest inside that drawn-up tarp now. In the winter they sleep on a ledge right outside the chicken coop. As part of my nightly ritual in the winter, of closing up the chickens, I shine my flashlight up there, and make sure there's several pairs of feet. They are cute little guys!

I have to chime in here too :) I've never seen the foil in the nest, though I have seen plastic. I read an article by a local wildlife writer who wondered if they nest near to humans to be 'protected' by our presence. It was a well thought out article and it makes me wonder too. It would be an interesting study to find out if they are trading the risk of being so near people over the protection from other animals that may be hesitant to come so close to our stuff. They definitily pick some odd nesting sites. We've had several nest in the garage, on the porch and one in our new, not finished house. Thanks goodness they fledge pretty quickly. A neighbor had one nest in her mailbox, she put up a temperary one for the mail until they fledged.