This is mostly common sense, but here's visual evidence that Haitians are gathering in the open spaces, places that probably should be transitioned to mass shelter areas run by agencies that can provide food, medical, and emotional support. Some are too small, but at least a few of these open spaces (stadiums) I know we pre-identified by looking at Visual Earth sat maps just after the EQ happened. http://www.digitalglobe.com/downloads/DG_Analysis_Haiti_Earthquake_13Jan2010.pdf

Getting aid to a population that already is sitting in the safest place possible is different than getting aid to people who remain in debris-filled streets. You need a path to the site; you need to begin with announcements to those on the site of the plan for a shelter, negotiate for space with those already on the ground if necessary, and begin to offer services, water, food, medical care, transitioning people over the a shelter space. The shelter space should generally be behind a perimeter that can be protected, and services continued there. ICRC and the UN has relevant experience in doing this in mass disaster scenarios. Like Arney points out, the nature of people doesn't change immediately after a disaster, social bonds strengthen, and the prospect of an active shelter and recovery area with an offer of assistance is generally enough to get people involved in setting it up and receiving services. You do need to be careful about a stampede, because each shelter space can only accomodate so many people, and a population can descend on a fresh site from miles in any direction. The sheer number of affected Haitians is scary. Again, there's a helluva lot of logistics to consider in deliverying aid there.