I am soooo thankful for this thread. I was out of state and running with the salt lake city running club on a 7 mile run through a neighborhood. My gut started rumbling, so I let them go ahead and hoped I could walk it off. There was no where to duck into. I found a small house turned into an accounting business, so I ducked into the side of the house behind the garbage cans JUST IN TIME! I could see into the neighbors living room window. MOST EMBARRASSING MOMENT. Everyone wanted to know what happened to me at the end of the run.
Now I do my first two mile around my block before heading out. I'm glad to get this off my chest.
This happens to many, many people. Immodium is moderately helpful (night before and morning of the long run and/or race), but a well-placed bathroom (mile 1, 3, 5, whatever) is the best solution for me. I have used pine needles, though.