Siri currently works with data from Yelp and the knowledge engine Wolfram Alpha, but developers are eagerly anticipating the chance to make their own services Siri compatible.
RTM is apparently using a simple workaround to take advantage of Siri: It involves creating a new CalDav calendar on your iPhone 4S and making it your default reminders calendar. So now when you ask Siri to make a reminder for a certain point in time, it will get added to Remember the Milk instead of showing up in the iPhone’s “Reminders” app. That means you’ll have to rely on the RTM app to keep track of your reminders, instead of Apple’s built-in app.
By making Siri work with RTM, you’re also disabling iOS 5’s normal reminder notifications. I tried creating a RTM event using Siri and it successfully made its way to the service, but my iPhone didn’t remind me on time because I don’t have the RTM app installed. For heavy Remember the Milk users, this shouldn’t be a problem, but I think the company would do well to remind users of the way it changes the iPhone’s built-in reminder service, as well as the fact that an RTM app is necessary.