In August of 2006 AOL leaked millions of search records. Some SEOs scoured through this data to look at click data by ranking. A comment on Jim Boykin's blog reveals the percent of clicks for each position for 9,038,794 searches and 4,926,623 clicks. Donna Fontenot shared the relative click volume of lower ranked results relative to the top ranked site.
4,926,623 / 9,038,794 = 0.545053134
So 54.5% of searches on Google result in a click. Alternatively, 45.5% of search do not result in a click. I think this is an important data point to keep in mind when dealing with metrics.