A Little Background Information:
I\'ve been looking at a few PHP framework recently, and it came down to two. The Zend Framework or CodeIgniter.
I
I did a quick google search and found the following: http://devcha.blogspot.com/2007/12/smarty-as-template-engine-in-code.html http://codeigniter.com/forums/viewthread/67127/
If the designer is not familiar with Smarty, I think it's almost the same as if you use the existing CodeIgniter templating system (which leaves everything to PHP actually). It also depends on the complexity of the project at hand.
You can also hook Smarty with Zend Framework. It's more complex than with CodeIgniter, but there's already a primer on how to do exactly that in the ZF documentation. http://framework.zend.com/manual/en/zend.view.scripts.html Plus lots of tutorials on the net.
In my opinion it's almost the same, you can use pure PHP or Smarty as your template "engine", so it depends on the project. Also, compare a developer who has extensive experience and already has a library of view helpers so she uses pure PHP, versus a designer who doesn't know anything about PHP, but has extensive experience with Smarty. Sometimes decisions have to be based on who is going to do what.