At the moment I am reading a fantastic book called 'Blackwater: The Rise of the World's Most Powerful Mercenary Army' by Jeremy Scahill. It is a fascinating account of the rise and rise of Blackwater, the private security firm run by ultra-neo-theoconservative businessman Erik Prince. Involved heavily in Iraq and Afghanistan as 'private contractors' - read 'mercenaries' - this company has the largest privately funded and recruited private militia in the world.
It now seems that Blackwater and contractors in general have become a political hot potato in the current US presidential race, with Scahill pressuring both Clinton and Obama into taking a stance on contractors: namely, that they should be phased out over the coming years. Obama's position seems more reticent than Clinton's, but both seem to be moving in the direction of attempting to curb the involvement of security firms such as Blackwater in future conflicts overseas.
Here is Scahill talking about the rise of Blackwater on The Nation: