I can successfully svn up from that repository on the command line. Pointing to that spot in the tree will get you a lot of files. Maybe your connection is experiencing latency problems or something. You could try pointing to a directory that is lower in the tree, like http://svn.alfresco.com/repos/alfresco-open-mirror/alfresco/HEAD/root
You might also try the command line rather than subclipse but I doubt that's the problem.