i tell this to everyone who gets drug tested - i smoke weed occasionally, around once every month or two. i have smoked weed less than a week before a piss test, and passed.
the only way you will fail a drug test for pot is if youre a heavy smoker (every day or so)
the end
edit - its funny, weed stays in your body for up to a month, but coke is gone within a few days. so you can pretty much blow a bunch of yay one day, then 3 days later piss clean. but people get in trouble for smoking a little weed every now and then. doesnt make much sense.