While this will certainly change the way Metro police in DC handle marijuana possession (now you'll get a beating and a ticket, instead of a beating and thrown in jail), it's worth noting that DC is covered with Federal property that is under the jurisdiction of Federal law enforcement including the FBI, Secret Service, US Marshals, Smithsonian Police, US Park Police and Capitol Hill Police (to name just a few that come to mind). If residents of the District are busted by any of them
on Federal property, it's a felony just like it's always been.
EDIT: sorry - it's not just on Federal property, it's by a Federal agent. I guess there's a question about jurisdiction for some of the above agencies, but not all.