It heavily depends on which services you want to add. Maybe you give some details on that.
Usually big services such as google, yahoo and such allow you to use their API and they have setup the appropriate headers (Access-Control-Allow-Origin, Access-Control-Allow-Methods and Access-Control-Allow-Headers)
If you control the different services yourself, you need to support a so called pre-flight OPTIONS request that basically sends the details of what you are allowed to do. This is implemented in the newer browsers. Without that or if it fails the browser usually falls back to a UNAUTHORIZED response which you cannot programmatically differentiate from an UNAUTHORIZED of the server.
This mechanism is supposed to protect users from accidentally being spied on. Since both the application (ember in this case) and the server need to clearly define what is allowed and what not.
With Ember.$.getJSON you can invoke/parse JSONP callback which is why it might serve you well. In some of my projects I used bower package ic-ajax as it is more Ember friendly (it wraps the jquery in a promise API).