They voted for it twice before in '96 and '98, as Jacob Sullum explained, and though it looked on election night that the third chance for Arizona voters to get medical marijuana had failed, it's still in play, as Drug War Chronicle reports:
though precinct reports had been completed, 290,000 late mail-in ballots (mailed at the last minute or dropped off in their mail-in envelopes at the polls Tuesday) remained to be counted, as did 84,000 provisional ballots (accepted at the polls but with the voters' eligibility in question), according to a press release from the Arizona Secretary of State's office. MPP pointed out that a 52%-48% split in favor of Prop 203 among those late-counted ballots would make up the roughly 7,000 vote difference between the yes and no vote counts.
The question then for Prop 203 partisans was whether those late mail-in and provisional voters are representative of the state as a whole (e.g., the equivalent of a random sample of Arizona voters, in which case one would expect the same statewide split on the initiative), or whether they differ demographically from the state as a whole, in ways that affect that split. A late Thursday afternoon update on the Secretary of State's web site suggests good news for the measure on that front, with an improved 49.86% voting yes on 203, 65,718 additional votes having lowered the vote difference to 3,870. Assuming those votes were mail-ins or provisionals, not additional precinct reports, Prop 203 at that rate could reach 50.4% or even 50.5%.