Another -1 for Smarty. PHP itself works great.

Smarty is another layer and if you're not the only one doing is an this is going to be for users to develop templates/themes it's more support for you as they'll need to know a bit of PHP, HTML and Smarty....

If they just know HTML and learn a bit of PHP, they can get a lot done.

I know with the Gallery project used Smarty in version 2 and in version 3, they've done away with it and have moved strictly to PHP. For the better too, it's much easier to work on a template or theme in this manner.