When wiring up an Ext ComboBox as an ajax typeahead control, it was not obvious to me how to get the list to display a message when no results are returned by the server (the default behavior is to just roll up the list and give no user feedback).

The trick is to use the following in the config options:
listEmptyText
http://www.extjs.com/deploy/dev/docs/?class=Ext.form.ComboBox

Don't be confused by "emptyText" or "invalidText" properties!