Raymond Chen's 2012 year-end link clearance includes one to a New Yorker article about a man who seems to have gone to considerable pains to record very good marathon times without actually running the whole races. In the age of transponders and multiple mid-course timing mats, this would require a great deal more effort than it would have 30 years ago.
I took a short-cut in a marathon once, the Marine Corps Marathon of 1980: so did all the thousands of other runners, those anyway who made it a little more than two thirds of the way. The course was shortened in two places: first, when the lead runners followed the press truck in cutting off the very tip of Hains Point; second, when runners were directed to cut across the grass near the Jefferson Memorial. I rolled out of bed the next day quite happy with my time, read the news, and wished to go back to bed and sulk or mourn. But I believe I went to work instead.
As for transponders, I am now suspicious of them. In the last race I entered, the electronic results showed me finishing many places behind where I did. I'm guessing that in the hurry of handling the last-minute registrations the staff mixed up name and transponder number. Well, the time I actually managed wasn't worth arguing about.
January 3: I should add that we who finished the 1980 Marine Corps Marathon received a certificate with two times on it: the actual finishing time, and the estimated time over the standard marathon distance. As I recall, the shortcuts amounted to about 500 yards.