Developer Brandon Fiquett created the hack based around a proxy server and a ruby plugin.that allows him to send commands to his car. At present, Siri can unlock and lock the car, open the trunk, and also start and stop the ignition.

Siri will currently respond to commands such as "Vehicle Start", "Vehicle Stop", "Vehicle Pop Trunk" and the slightly more conversational "Start my car", "Lock my car", "Pop my trunk".
Of course this won't work for everyone's car--Brandon's vehicle has Viper SmartStart installed so it can be controlled remotely. Therefore, models without any form or remote controlling won't be able to benefit from the tech. If you can remotely control your car, though, this could in future be good for those who frequently forget their car keys, or perhaps can't reach their keys at the bottom of their bag.
Personally, I think it'd be impressive when the likes of Siri will drive the car on long journeys for you, once the designated driver begins to feel tired (or you just don't feel like driving that day). Granted, we're probably a little ways off from that--unless Google gets its way.
Here's a video of Brandon showing you Siri starting up his car's engine. Head over to his blog to see a couple more of the technical details, including the resources needed, if you are considering trying this yourself.