Well, that is if you have a car with a Viper remote start system. Developer Brandon Fiquette has written a PHP script that would send notification to the remote start service and ultimately, start your car.
The hack was made possible by the Siri Proxy Server. The proxy server allows the custom creation of handlers – such as the included plugin, which allows you to adjust my thermostat at home.
With this software, you can create other handlers that could turn Siri into a full remote control. Of course, you need devices that would be remote controlled or internet accessible.