If California legalizes cannabis, how will the decision affect pre-employment/workplace drug-testing?

I think that it is small issue. It is not a problem to get a note from your doctor saying why you test positive for Vicodin when you are on it, so there shouldn't be a problem with getting a statement. If there is a problem with working somewhere just because of the medicine you are prescribed, I'd say find yourself a good lawyer, and sue the pants off of that said employer.

And you don't have to smoke it either, this safest medicine, Marijuana, can be eaten as well.

Yes, pot is illegal in Texas. If you fail a drug test in Texas, you could be fired. There is no law that states that an employer has to fire you for failing a drug test, however.

Companies could still require that people be free of this drug, and test for it. There are "smoke free" companies out there (American Heart Association and the American Lung Association) where you can't smoke tobacco so why not marijuana?

testing would probably only include impairment while working, just like driving....