I am not familiar with WebObjects but since it seems to provide a web application framework that can run on top of J2EE compliant application servers, there might be a way to bridge with Portal using the servlet/JSP support of WebObjects.
Take a look at http://developer.apple.com/documentation/WebObjects/JSP_and_Servlets/index.html. One possible solution would be to expose your WebObjects application as a servlet then bundle that servlet along with a portlet that would integrate with JBoss Portal. Obviously, the details of how this would concretly be done need to be worked out... and cannot really be answered abstractly.
The short answer to your question is support for WebObjects is currently unknown but should be possible via a bridge effort. I'd be interested in learning about it if you manage to get it to work.