i work at a large national drugstore chain and in addition to having a drug screening before you can work there they also have random drug tests where the testing service comes to the store and tests everyone (if you leave you get fired) i've worked there for several months and this hasn't happened yet but i know that if it ever did most of the employees woud get fired.
i agree with the restaurant/ups idea --i know people who work at both and no drug tests. anyway, you will get paid more working at those places than you would at walmart.