For non liquid templating system, you can try theme_support_plugin. There has been hacks for this to be able to work with Rails 2. For liquid, you can check out more info here: http://www.artweb-design.de/2008/2/5/sexy-theme-templating-with-haml-safemode-finally
Have a look at liquid template rendering by mephisto blogging application. You can also view working of CMS such as Radiant which too uses liquid templates.