I know you are looking for UML, but if you are looking for something to show the (partial) flow of your webapplication, you can take a look at what I've done at:
http://members.home.com/dlongmuir/struts.html
The main problem with it is that it doesn't parse the jsps to look for links, so basically every JSP is a leaf node.
Anyways, it is a work in progress (slowly), and other people have said that it helped them out.
If you decide to try it, there is one gotcha - the latest version of Xalan doesn't like my xsl. If you try it with an older version of Xalan, then everything is fine (I really did generate the pictures on the page! :-) ).
Derek.
