SmartyPress is another Smarty driven WordPress project.
It looks interesting, and I had the thought that it might be possible to paste an alternative template system into WordPress by piggy-backing on top of the current system.
The home.php, single.php, index.php files of the various themes could be loaders for another more secure templating system rather than PHP. It would certainly make editing templates and securing them against abuse easier, although for WordPress.com we have even fancier ideas to be announced later!
"SmartyPress - Smarty Driven WordPress",