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:
See this working version (and files to download) on my web site: