tedster - 9:53 pm on Dec 17, 2009 (gmt 0)

Welcome to the forums, Peter.

Probably the best aggregate data available came from the now infamous AOL leak back in 2006. It's a bit old, and Universal Search has definitely messed with the accuracy, but at that time the data showed:

#1 - 47% click-through
#2 - 13% click-through
#3 - 9%click-through
#4 - 7% click-through

That is aggregate data across many types of query terms, and it will vary quite a bit by the nature of the query and even the specific SERP competitors, but it's a decent ballpark.