Pacemaker-hacking feels like a Hollywood plot. Assassinate someone by taking over the medical device that controls his heart? Surely that isn't possible in the real world.
Actually, it's totally possible. Last year, a researcher demonstrated a pacemaker hack in Australia, with a virus that could conceivably spread from one person to every electronically assisted heart within 30 feet.
The threat level
Probably limited to assassination targets. Think: high-profile people, who have pacemakers. It would be an elaborate attack, though, since you need more than just an internet connection to pull it off. Odds are that anyone interested in doing this could get a bomb or a pistol to the target cheaper and faster. This is Hollywood not because it's impossible, but because it's not very efficient.