According to MarathonGuide.com, here's the time for the 20th, 50th, and 100th finisher at each marathon in 2008 (or 2007 if it's a fall marathon). Races arranged by the 20th place time.
Chicago (2006) - 2:15, 2:20, 2:26
New York City (2006) - 2:19, 2:28, 2:35
Boston - 2:23, 2:29, 2:35
Grandma's - 2:27, 2:45, 2:58
San Diego RnR - 2:30, 2:51, 2:59
Arizona RnR - 2:34, 2:45, 2:56
Twin Cities (2007) - 2:34, 2:48, 3:01
Los Angeles - 2:34, 2:51, 3:09
Philadelphia (2007) - 2:36, 2:44, 2:51
Houston - 2:36, 2:48, 2:58
Dallas White Rock (2007) - 2:39, 2:53, 3:04
Austin - 2:40, 2:51, 3:00
*Chicago 2007 was anamoly with the heat.
*New York 2007 might be skewed due to fast runners being in the Olympic trials the day before instead.
Others?