They can still fire you. If it is in your system how do they know if you smoked last night or smoked right before you came to work and got into an accident? That is the same as if you had alcohol in your system. Alcohol just flushes out quicker.
There is a hospital in Tennessee that does nicotine tests on employees and if you have it in your system they terminate you.
I don't believe it will be so cut and dry. I also think policies are going to vary wildly for the next year or two assuming the Fed doesn't drop a serious hammer on it.