Now, Diaz will most likely have to serve a one-year suspension, and could lose a portion of his purse from the Condit fight. Diaz previously failed a drug test in 2007 after a win against Takanori Gomi. At the time, he was fined 20% of his purse and suspended for 6 months.
Now, UFC President Dana White will have to make a decision about Diaz's future, assuming he reverses his retirement. Diaz isn't shy at all about his marijuana use, and actually has a card permitting him to use it in the state of California, due to his attention deficit hyperactivity disorder.
Unfortunately for Diaz, that excuse doesn't fly with the Nevada State Athletic Commission. Executive director Keith Kizer said they'll be seeking disciplinary action.
The best guess: Diaz will probably lose 40% of his $200,000 purse for the fight, and he'll likely be suspended for a year. The precedent was set when Matt Vanda was caught with marijuana in his system for a second time in New Jersey in November of 2010.
If Diaz wants to fight again, I'm sure White and the UFC will be more than happy to let him. He's an outstanding fighter, he's a fan favorite, and he's still a legitimate title contender. At some point, though, Diaz is going to have to figure out how to work things out so he can stay in the octagon without getting suspended again.