For a project I am working on I wanted to add a tree to a viewport. There is a portal example on the Ext web site that features an accordion control in the west panel. It took a while to figure out how to replace that accordion with a tree even though the method turned out to be quite simple.
See this post on the Ext forum for the key piece of information from another Ext user:
http://extjs.com/forum/showthread.php?t=14445
See this working version (and files to download) on my web site:
http://www.murrah.com.au/samples/ext/portal/portal.cfm