Just a guess and not official, but based on past marathon events I would think the 2009 dates would be 1/10 for the half and 1/11 for the full. Typically the weekend before the Martin Luther King holiday weekend, and in 2009 MLK day is on 1/19/09.
For the past few years--it is the weekend following New Years....so I guess it is the one weekend in January with nothing associated with it (well until they moved the Superbowl to February).