Mind Hacks has a new article about performing multiple cognitive activities at once. (Second link to Mind Hacks in as many days. Hm.) It reminds me of this story, told to me (and the rest of the students in class) by Herb Simon, Nobel Laureate and teacher of a Cognitive Psych class I took.
Herb flew to Buffalo (or somewhere) and was picked up at the airport by a colleague. As his colleague drove them into town along the snow-covered roads, they discussed whether the human mind could process tasks in parallel. Herb argued that it could not, and gave his driver a set of instructions. "Picture a rectangle that is twice as long as it is high. Now bisect it diagonally. Slice it vertically two-thirds of the way from the left edge," and so on. Herb told us that as the number of instructions increased, the car went slower and slower. He delivered this punchline: "Fortunately, when we went off the road we were going so slowly that there was no damage to the car."