Remember the Milk is an online task management system that lets you create and sync tasks with your mobile devices. If you have the iPhone 4 or earlier, Remember The Milk users can install an iOS app that'll sync their tasks for them. But if you have an iPhone 4S, you can use Siri to add tasks your RTM account.
Apple has not released an official API so the developers at Remember The Milk have created a clever work around. They use CalDav to push your reminders to RTM. If you have the latest iPhone, you can point your browser to RTM's website and follow a few simple steps to setup this feature on your iPhone 4S. Once you have CalDav in place, iPhone 4S users can use Siri to create a reminder and it will be sent automatically to your RTM account. Check out the video below to see this work around in action.