What search engine do you need?
If it's for some search on the web, there is the GooglePortlet that is available here : http://sourceforge.net/project/showfiles.php?group_id=91153
To make it work, you also need the Google key that you can dowload here:
After, you need to create the *.xml files for JBoss Portal and put the value of the Googlekey into portlet.xml like this:
<init-param> <name>key</name> <value>YYY+XxxXXXXXxxxxXX+YYy</value> </init-param>
I wish it helps you and it was an engine like that you need... if not, sorry for the disturbance.
i guess g_nadia thinks of a search engine that indexes all sites of the portal and provides a portal wide find dialog. I have heard something like that could be done easily with a portlet based on apache lucene.