The United States Patent and Trademark Office just grated Apple a patent for a new system which broadcasts the user's call availability. Unlike other systems of this kind, Apple's will use a wide variety of contextual information to automatically determine the customer's willingness to answer a phone call. This service should be available on-demand for anyone who wants to check before actually bothering someone and each user will have the option to make himself / herself permanently available to specific people from the contact list.
This service will use a wide variety of information such as ringtone volume, current time zone, location, battery life, signal strength, etc. to automatically determine if you are available to take calls. Basically, if you are in a meeting, at the opera, or in other places where you wouldn't want to be disturbed by a phone call, the system should automatically let others know that you are unavailable. Of course, this will only work if those who want to call you bother to check your availability and if they actually take your status into account, so its efficiency is quite iffy.
Even though I don't like it very much when companies can make predictions about my activities by studying the current condition of my phone, I must say that this idea sounds pretty interesting. However, I think the Apple will have some big problems if they won't make this an opt-in service. Furthermore, the tightness of the sharing controls will probably also have a huge impact over the users' approval.