Using the s:url Tag in a portlet environment portlet urls are generated.
However, to serve resources you have to use servlets with portlets v1.0.
Its not possible to generate a servlet url with a s:url Tag when served through the portlet dispatcher.
You can only hardwire your "path" to your servlet action using the s:url value attribute, but its not possible to use action and namespace definitions.
A new portletUrlType to specify the type of url would be nice to make use of servlets.