Are you able to put your computer in the DMZ temporarily then reload Eclipse and see if the XML editor is effected?
I believe there is another thread in the forums (I thought it was this one) talking about a authenticating http proxy problem causing major pauses with validation on the XML files; I wonder if you are running into a proxy problem as well?
Also, if you click the url http://jakarta.apache.org/struts/dtds/struts-config_1_1.dtd from the same machine you are running Eclipse on, does that link load anything for you? Scott mentioned that ME uses straight HTTP connection to pull down the DTDs for the XML editor, so if you can load that from a web browser you should be able to see it in ME.
Also if it DOES load up in your web browser, double check if you have any proxy settings in your browser set; that might explain it.