A quick Google perusal indicated closed registration the 2nd week of June in 2009 and half-full April 23, 2008. I'd recommend waiting no later than April 1 if you're fairly serious about entering. Keep in mind the trend has been for races to fill faster and faster.
I have seen craigslist ads for black-market bibs the last 3 years (from people who did have to commit ridiculously early and got injured/realized they were undertrained)/had sudden obligation come up), but the ads aren't long-lived (?? proportions of being flagged or someone quickly making the buy).
There are now several more regional marathon options that have popped up in the last 5 years that won't require you to pay through the nose months in advance. While not the org, course, and "prestige" of TCM, there are some other options that time of year, incl. ones in Milwaukee, Ashland, Wi. Dells, and Middleton (nr Madison), WI; Mankato, MN; Des Moines, IA. Check marathonguide.com for more.
Good luck and have fun if you come.