With alcohol, there are ways to test if one is "under the influence" - because after a relatively short period of time, it's out of the bloodstream.
As far as I know, right now, there is no consistent, viable way to test for "under the influence" of THC, because it stays in your body for weeks.
And "Dorito-Fingers" is not a reliable test.
But I suppose employers can come out with a "No Pot" policy, just as hospitals have come out with a "No Tobacco" policy:
effective January 1, 2012, UH will no longer hire external candidates who use any form of tobacco products. External candidates will be required to confirm their non-tobacco use when expressing interest for a job opportunity. Those selected as a new hire will be tested for all tobacco use as part of the pre-employment lab screening process. Candidates who fail the tobacco screening may re-apply after 90 days.
I'm sure there is some grandfather clause for current employees, and incentives to quit.
Where I work, tobacco users pay more for their health insurance. But I haven't heard of anyone being tested for tobacco .. and I digress...