Jacob Sullum explains what happens now in Colorado and Washington state:
The elimination of penalties for possessing up to an ounce of marijuana (if you are 21 or older) takes effect right away in both states (once the governor proclaims/certifies the results, within 30 days of the election). But the provisions allowing commercial production and sale of cannabis for recreational use require regulations that will be written during the next year. The Washington Liquor Control Board has until December 1, 2013, to adopt regulations for marijuana growers, wholesalers, processors, and retailers. The deadline in Colorado, where cannabis businesses will be overseen by the state Department of Revenue, is July 1, 2013. Colorado's law, unlike Washington's, also allows home cultivation of up to six plants and nonprofit transfers of up to an ounce, so Colorado pot smokers will have an immediate state-legal source of marijuana.
John Paul Rathbone adds:
To get a bead on what this might mean, this is further than Netherlands has gone. There, contrary to common perception, it is only the retail sale of 5 grams that is legal. Production and wholesale remains illegal, and the law is vigorously enforced. That is why the price of pot in Amsterdam “coffee shops” is “little different than the price in US dispensaries,” as the authors of “Marijuana legalisation: what everyone needs to know”, argue here.
Yglesias foresees a lot of federal and state conflict:
[B]efore you buy land to start your marijuana farm, note that the drug is still illegal in the United States of America and that Colorado is one of the United States of America. Consequently, if you try to set up a large-scale pot business you're liable to get busted by the DEA. So expect a lot of clashes around this and a sticky situation for the Obama administration.