For a number of years, there has been a provision in the U.S. federal budget -- known variously, among other names, as the Rohrabacher-Blumenauer Amendment and the Joyce Amendment -- that denies funds to the U.S. Department of Justice to prosecute conduct that is in compliance with state law regarding medical marijuana. 

The text of the amendment reads generally as follows:  "None of the funds made available under this Act to the Department of Justice may be used, with respect to any of the States of Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, and Wyoming, or with respect to the District of Columbia, Guam, or Puerto Rico, to prevent any of them from implementing their own laws that authorize the use, distribution, possession, or cultivation of medical marijuana."

Although the president's recently proposed budget does not include this amendment, it is hard to imagine any budget ultimately being passed by Congress that does not include it.  Congress has insisted on its inclusion before, and it is hard to imagine Congress backtracking now.  Nothing should be taken for granted, however.