Crimes reported to the police and total crimes committed in a calendar year will vary based on the reporting method and on definitions. For the purposes of this answer, the FBI's definition of violent crimes is used:
...violent crime is composed of four offenses: murder and nonnegligent manslaughter, forcible rape, robbery, and aggravated assault.
Based on that definition, as collected and reported by the FBI, the number of violent crimes reported in the U.S. in 2009 would be as follows:
In 2009, an estimated 1,318,398 violent crimes occurred nationwide, a decrease of 5.3 percent from the 2008 estimate.
The apparent decrease in violent crime statistics could mean that crime-prevention methods had increased their efficacy, or that fewer crimes were reported to the police. Remember, when crimes are not reported, they are not logged and detailed unless they are exposed in a different investigation; it is also important to note that many personal crimes are never reported because of shame or guilt. Rape is a commonly-unreported crime, as the stigma is severe and the investigation often turns up more issues, while "nonnegligent manslaughter" could easily be excused as accidental. However, the violent crime trend was clearly down for a total of four years until the end of 2009, so it is very possible that prevention methods were working and fewer people felt the need or the impulse to commit violent crimes.