How to use smarty in Drupal 7
While there is a Smarty module, it has no release for Drupal 7 and I doubt there will be one.

For Drupal 4 and 5, there were various efforts made to use different template engines.  However, nearly all contributed themes for Drupal 6 and 7 are done in PHPTemplate and almost all documentation and available support are for PHPTemplate.

Even if you're already familiar with Smarty, if you're going to use Drupal seriously for more than one project I would highly recommend learning PHPTemplate.  At this point, you would have to do work to integrate Smarty into Drupal 7 on your own, which would almost certainly be harder than just learning PHPTemplate.  Going with Smarty will also limit your support options.