I have been experimenting with GWT using a RESTful (jersey server) backend as its data source. When deploying both my application and the backend to the same server this worked great, but once the server side was finished I wanted to deploy it to Tomcat on my Amazon EC2 instance, and continue debugging the application using GWT's Developer mode. This, unfortunately, does not work, due to Same Origin Policy restrictions of virtually all browsers.
The most standard way of circumventing the Same Origin Policy is to use Json with Padding, or JSONP. The technique in a nutshell is that the server returns a JavaScript function call. The client thas prepared for this by creating this function in the DOM of the current page in the browser. The client tells the server "The name of the function is xyzzy" by sending in a query parameter, usually named "callback." This post shows some ways around this issue.