The Portal displays a portal named "default" by default. I have not been able to determine if there is a way to change this (looking at the source seems to indicate that this name is hard-coded).
What I have done to create a new portal is defined the whole thing within a separate *-object.xml file. Then I modified jboss-portal.sar/conf/data/default-object.xml, changing the < portal-name > entry to something other than default. Also, in my *-object.xml file I modified the < portal-name > entry to be 'default'. I placed my *-object.xml file into the jboss-portal.sar/conf/data directory (yes, there are now two *-objects file in that directory). Finally, I deployed the jboss-portal.sar to a clean JBoss installation with a clean database.
I did find a solution, quite frankly by mistake. I looked to see what it was that defined a default page for a portal, and figured it was worth a try.
I am not sure how you would do through xml files etcetera, but I did find out that you could do it though the management portlet on the Admin Tab of the "default" portal.
Click on the root node. Click on properties. Add a new property named "portal.defaultObjectName" with a value of the new portal instance (in my example "newportal") Click update.
Now when you go to /portal you end up getting to the new portal instance.