The simple answer is, it will still be illegal for a user of marijuana or any other Federally controlled substance to purchase, posess, or transfer firearms or ammunition.
The ATF has been very clear on this and here is a link to the open letter to FFL’s.
http://www.atf.gov/files/press/releases/2011/09/092611-atf-open-letter-to-all-ffls-marijuana-for-medicinal-purposes.pdf