A statistical report from TRAC on federal prosecutions is out. It covers the month of August, 2008. It begins:

Greetings from TRAC. The latest available data from the Justice Department show that in August 2008 the government reported 13,566 new prosecutions. While down slightly (2.3%) from the previous month, this number is still 43% higher than a year ago. The overall growth in federal prosecutions is largely driven by continuing increases in immigration matters, which accounted for 54% of all new cases filed in August in U.S. Federal Court.

As for all prosecutions:

The largest number of prosecutions of these matters in August 2008 was for "Immigration", accounting for 54.4 percent of prosecutions. Prosecutions were also filed for "Drugs-Drug Trafficking" (14.1%), " Weapons-Operation Triggerlock Major" (5.7%), "Other Criminal Prosecutions" (3.3%), "Drugs-Organized Crime Task Force" (3.1%), "Assimilated Crimes" (2.8%), "Withheld by Govt from TRAC (FOIA challen" (2.7%), "Other-Not Specified" (2.2%). See Figure 2.