Accessing a web service is not different in a portlet environment than in a servlet environment.
Context-root still make sense if you need to specify one, your portlet will be located in a war file and then will be accessible from that context-root.
I would suggest that you start by writing a simple Servlet accessing a Web service. Then get a simple portlet running, you can get the HelloWorld from portletswap.com, then mixing the two wouldn't be hard. Move your Servlet service code to the render method and build from that.
Hope this helps.