Implications of Michigan's marijuana driving law?

The Michigan Supreme Court has upheld a law which makes it illegal to drive with any detectable amount of marijuana in your system. As Jacob Sullum points out, this is, kind of nuts. Marijuana stays in your system for days to weeks after you're no longer intoxicated. What I wonder about is the standard required for testing; in most states just by driving you're considered to have consented to chemical testing for alcohol. In the particular cases that are the the subject of this suit, there was evidence that the offenders had smoked marijuana, but I wonder if you can just be stopped and randomly tested.