I stumbled across this interesting report at MarathonGuide.com presenting marathon statistics for 2005.  The part I found most interesting was the ranking of marathons by number of registered finishers:

2005 Largest USA Marathons
Both New York and Chicago did not increase their registration limits in 2005 and so continued with their respective ranking as number one and two marathons in the USA. The LA Marathon and Marine Corps Marathon each added more than 2,500 finishers - and the Nike 26.2 (which can’t quite decide if it’s a women’s race or not) nearly doubled in size. Shaking up the top 25 rankings was the New Las Vegas Marathon, which as an inaugural event, debuted at number 8 and just missed the record for an inaugural event which was set by the Rock N Roll Arizona Marathon in 2004. In total, the country’s 25 largest marathons, accounted for more than 70% of total marathon finishes in the USA in 2005.