Assuming the feds are unable to crush legalization immediately, the legalization of pot in Colorado and Washington is predicted to cut the profits of the Mexican drug cartels. According to a another recent article on Mexico's reaction to the legalization:
About half of all marijuana consumed in the U.S. comes from Mexico, but it is not clear how much legalization of the drug in two U.S. states will impact the revenue of Mexican drug cartels. Eric Olson, deputy director of the Mexico Institute at the Wilson Center, said that revenue from marijuana comprises about 20 percent of cartels’ total revenue. A 2010 study by the Rand Corporation said that drug cartels derive about 15 to 26 percent of their revenue from marijuana sales.
However, a different study, by the Mexican Competitiveness Institute, said that legalizing marijuana in just the state of Washington could cut drug cartel profits by $1.37 billion, or 23 percent.
It's difficult to guess which of these sources are closer to being correct, but we do know that the end of prohibition policies are always bad for organized crime. This was dramatized in Sergio Leone's film Once Upon a Time in America in which the gangsters held a "funeral" for prohibition at a speakeasy and played a dirge in Alcohol Prohibition's honor. The end of Prohibition meant the end of easy money for organized crime: