For a comparison, Kosovo has 1.8 million people, comparable to metro Port-au-Prince's 1.7 million. KFOR at its height had 4 brigades and 50,000 soldiers, of which only 7,000 were from the US. Where will we find 50,000 soldiers (roughly 2 divisions) in the US without disrupting dwell time?
Security may not seem a concern right now because of the human suffering. However, as we have seen in Iraq, people get angry real fast, especially when comparing the perceived omnipotence of the US military with the reality. When the survivors start dying due to inadequate aid and gang disruptions, their relatives will be very upset. It will be sad if we see a repeat of Blackhawk Down, this time with another group of "skinnies".
G also brought up the potential immigration challenge. We almost definitely will have to dust off the camps at Gitmo. For some Haitians, it could be a return to familiar ground.
Some people in Galrahn's comments have suggested deployment of cell towers and other communication infrastructure stuff. Sadly, the US military has no money to spend on this kind of "side missions". Perhaps some state emergency management agencies have this asset available for deployment. If the USAID or State Department can organize this kind of deployment of American emergency management assets, that will be a big help to the Haitians, good practice for the states, and good precedence and practice for soft power deployment in the State Department.
Perhaps I need to get ready for a mobilization into Haiti myself...