A few years ago Elise Co from the MIT Media Lab was working on a luminescent raincoat. It had panels that would light up when they got wet, mirroring the pattern of the raindrops.
Interactive rain gear has since made it out of academia and onto the runway. The June 2003 issue of Wired featured a transparent raincoat from Prada that becomes opaque when it gets wet from rain or perspiration. Miuccia Prada says, "Every piece of clothing shapes your body but also the space around you, the emptiness around you. This raincoat, from our 2002 winter collection, plays off that divide. ... It changes the relationship between what's inside and outside."
I'm not crazy about the perspiration thing, but I really like the idea of clothing responding to environmental factors like rain in a whimsical way.