Can a developer make an app that can be controlled with Siri? Example: A commute planer app could take a start and destination from you, through Siri, and plan your trip. Would be very useful, especially when you're in a hurry.
As of now, there is no API for Apple developers to use to connect to Siri. That may or may not be changed in the future; see e.g. http://daringfireball.net/2011/10/iphone_4s for discussion.