They're already doing it as a carrot in most health plans (go to this website and tell us how you worked out, how many steps you took today, and we'll put FREE money in your HSA!), and also auto insurance plans (plug this thing in your OBD-II port, and we'll cut your insurance rates provided you drive in a certain way!). I'm exempt from the auto one, since I don't have an OBD-II port in any of my shitpiles, thank goodness.
It's not a far bridge to get from carrot to stick, since they're the ones that control pay schedules and rates.