Or marihuana smokers and dealers. But not alcohol or tobacco sellers.

Yeah, that guy who dealt marihuana on rollerblades in NYC could get tracked in a sting operation which would also catch most of his customers. All they'd have to do is to "tail" him with such a device over a month.

Right now, it would be way too expensive to devote such a system to catch one dealer, but the scary thing is that systems like this could surveil and process entire regions/neighborhoods as a batch, and the expense of doing so is driven by the cost of communications and processing power. However, we know those costs are going to go down. From this, we should be able to estimate the time when large metropolitan police departments will be able to afford such capabilities.

That gives a concrete deadline for interested parties who wish to act.

EDIT: To clarify -- that would be a deadline for when it will be too late not a deadline for when to safely act by!