Working with ex. .tpl files? Getting NetBeans to recognize most of the code (at least the HTML, JS, and CSS), and do code highlighting on it, is as easy as adding 'tpl' as an associated file type:
UPDATE 24.03.2011: NetBeans have had a Smarty (.tpl) plugin for a while, but it hasn't been very good. Now it's getting much better. You can download it from withing NetBeans, go to Tools > Plugins > Available Plugins, search for smarty, check the checkbox beside the plugin and press Install.
(Make sure you have NetBeans Beta Update Center (See Settings tab of Plugins window URL: http://updates.netbeans.org/netbeans/updates/6.9/uc/final/beta/catalog.xml.gz for NetBeans 6.9))
NetBeans' blog posts about Smarty plugin: http://blogs.sun.com/netbeansphp/entry/initial_support_for_smarty_available and http://blogs.sun.com/netbeansphp/entry/smarty_update_custom_delimiters