Most people I've talked to know nothing about the fact that the U.S. administration intentionally sent firearms to Mexican drug cartels. I had read about it beginning around December 2010 and didn't understand how this was not "front page" news. It finally started getting covered by mainstream media in May 2011. In my opinion, the coverage of this government corruption is still not given the emphasis that is warranted.
In short, the U.S. administration intentionally sent firearms to Mexican drug cartels under the pretense that they wanted to monitor the flow of firearms even though they really had no effective means for such monitoring. These firearms have lead to increased violence, including the death of U.S. Border Patrol agent Brian Terry. Administration officials have lied about their knowledge of these operations.
Here is an extensive timeline of events that have been made public thus far:
Reasoned Politics: Hillary, Eric, Wikileaks and the BATFE Scandal: An Overview and Timeline