An interesting item on Stratfor. The bottom line: “It has now become quite common to hear U.S. officials confidently assert that 90 percent of the weapons used by the Mexican drug cartels come from the United States. However, a close examination of the dynamics of the cartel wars in Mexico — and of how the oft-echoed 90 percent number was reached — clearly demonstrates that the number is more political rhetoric than empirical fact.” I think there’s been some discussion of this on the Conspiracy in the past, but I thought the Stratfor piece was detailed and helpful enough to be worth passing along.
UPDATE: As commenter BFA points out, the BATF officially endorsed the 90 percent figure, writing in a report to Congress that “Over 90 percent of the crime guns used in Mexico originates from sources in the U.S.”