Well, I suppose you could say that if you take govenrment money you are giving implied consent for the government to make sure it is used for legal purposes, but I don't think you could make a very strong argument. If I drive drunk, they still have to have reason to stop me before they can get me to take the breath test. If someone has a history of drug abuse, there might be cause to have them tested regularly, but drug testing with no reason to believe they are drug users is an invasion of privacy and an illegal search, IMHO. Like you said, being in a rehab program or as a condition of employment is a separate issue.