Internet of Things: Security for Cars (and other critical stuff)

Security is a major concern regarding the internet of things. If your car's software can be updated remotely, what kind of risks does that create for malicious tampering? Or to take a less dire example, if your thermostat can be set remotely, what if a hacker tries to alter your setting? I have a couple of thoughts on safeguards. 

For less acute risks, such as the thermostat, every device should have a physical disconnect switch. So the person with physical control of the device, can instantly, indefinitely disconnect it from the internet. For more serious risks, such as autos, the ability to perform a remote update needs to be controlled. No wireless--software updates should only be installable via a physical port.