The 2011 Boston Marathon had 22,000+ entrants in one day to run 26 miles in April 2011.  Does this suggests changes (faster qualifying times or larger fields or both, and other things) are probably in store for 2012?  This also means that you better have a qualifying time by October 2011 if you want to run Boston in 2012.

Runners were allowed to start signing up for the 2011 marathon at 9 a.m., and the registration window closed just after 5 p.m., meaning about 21,000 people filled out their forms in eight hours. That's 65 times as quick as last year and more than 200 times as quick as five years ago, when it took almost seven months to fill the same number of spots.

Here's how long it has taken, since 2006, for the Boston Marathon's field to fill up.
2011 Oct. 18 Oct. 18 1
2010 Sept. 9 Nov. 13 65
2009 Sept. 3 Jan. 25 144
2008 Sept. 5 Feb. 26 174
2007 Sept. 6 Mar. 14 189
2006 Sept. 7 Mar. 29 203

Source: Boston Athletic Association from WallStreet Journal Article