To check your car's air pressure, you could get an air pressure gauge, or kick the tyres, but that wouldn't be quite as geeky as this aftermarket tyre pressure monitor. Four caps with sensor read each tyre's pressure and relay it wirelessly to the dashboard-mounted LCD. The setup also informs you if your tyre rapidly deflates, alerting you of a flat. Coo, but two problems: If the caps are indeed monitoring pressure, that means they depend on jamming a tyre's internal valve open, meaning that your tyres are going to need refilling often. Secondly, you are definitely going to lose these things.
AU: I agree you're likely to have them stolen, but I doubt they would contribute to faster air loss than usual. If they've been designed well, that is.