Hi,
I have 2 or more portlets on a page that I want to use a shared js library for that exists outside of the portlet structure.
If I add it to the jboss-portlet.xml for each:
<script type="text/javascript" src="/../shared/js/jquery/jquery-1.2.6.min.js"/>
If you're using a custom theme (or maybe even if you're not), I'd recommend adding jQuery to your theme directory. That way, you can maintain it there and reference it from all of your other portlets as necessary.