In 5 years (maybe less), I think this will be a non-issue. The NCAA already said something along the lines of "we have determined marijuana is not a performance enhancing substance" or STTE over these past few months. I would assume they did so in preparation for the inevitable failed marijuana test in Colorado or Washington, where it's legal for everyone else who isn't a NCAA athlete to smoke the stuff. (Which would be just another issue to be used against the NCAA in class actions suits ---- "see they treat us differently from the rest of the students...we should be paid like the employees we are".)
With Colorado estimating annual taxation of $125 million per year on cannabis sales, you can bet that this "pot of gold (or green, if you will) at the end of the tax rainbow" will cause more & more states to put legalization on the ballot. Within 5 years, I would think it will be prevalent enough that the NCAA won't ask for drug testing for marijuana anymore. And honestly, I wouldn't be surprised if it happens within 2 years.