UK Channel 4 has a good short video report on the security challenges of delivering assistance (first video on the page).

Also, from DoD:

Security Role in Haiti to Gain Prominence, Keen Says

By John J. Kruzel
American Forces Press Service

WASHINGTON, Jan. 17, 2010 – The security side of U.S. humanitarian relief operations in Haiti will take on a larger role as violence increases in the aftermath of the magnitude 7 earthquake that struck five days ago, the top U.S. commander in Haiti said today. Video

In the midst of the massive international relief effort there, Army Lt. Gen. P.K. Keen said some incidents of violence have impeded the U.S. military’s ability to support the government of Haiti.
“Our principal mission [is] humanitarian assistance, but the security component is going to be an increasing part of that,” he said today on ABC’s This Week. “And we're going to have to address that along with the United Nations, and we are going to have to do it quickly.”
Keen said they would monitor closely the "increasing incidents of violence."
"We do need, obviously, a safe and secure environment to continue and do the best we can with the humanitarian assistance," he said on Fox News.
and, from the National Post:

Security crucial in Haiti aid effort
Sheldon Alberts, Canwest News Service
Published: Friday, January 15, 2010

WASHINGTON -- A U.S. army brigade of 3,500 soldiers from the 82nd Airborne Division will join more than 2,200 U.S. Marines in Haiti by week's end as worries mount over the potential for post-earthquake unrest in a nation long beset by violence, drug crime and gang warfare.

The deployment ordered by Barack Obama, the U.S. President, is the U.S. military's largest to the Caribbean nation since September 1994, when several thousand Marines landed in Port-au-Prince to return exiled president Jean-Bertrand Aristide to power.

But even as the Pentagon rushes to meet urgent security and humanitarian needs, leaders from the United States and other Western nations, including Canada, are grappling with a bigger question: Will the massive international response to the earthquake mark the start of a long-term commitment to prevent Haiti from sliding once again into crime-ridden chaos?

"All of the effort is in saving lives right now and that's as it should be. But even while you have all your attention into saving lives, you've got to be planning for a much larger security apparatus for weeks and months to come," said Kara McDonald, a Haiti expert with the Council on Foreign Relations in Washington.