This isn't directly supported right now, without writing a third party website that Siri can hook into. According to the below linked documentation, from Apple's website, Siri on iOS 6 will support this functionality in at least limited part:
http://www.apple.com/ios/ios6/siri/
Eyes Free
Apple is working with car manufacturers to integrate Siri into select voice control systems. Through the voice command button on your steering wheel, you’ll be able to ask Siri questions without taking your eyes off the road. To minimize distractions even more, your iOS device’s screen won’t light up. With the Eyes Free feature, ask Siri to call people, select and play music, hear and compose text messages, use Maps and get directions, read your notifications, find calendar information, add reminders, and more. It’s just another way Siri helps you get things done, even when you’re behind the wheel.
This encourages me to believe that they will also expose the API (because someone will ferret it out if it exists) to normal API consumers during the iOS 6 lifecycle, probably before iOS 6.1, or with that release.