What is a front page in drupal ($is_front)?
$is_front is set on any template from _template_preprocess_default_variables() (itself called from template_preprocess()). It uses the result of the drupal_is_front_page() function.

drupal_is_front_page() will returns TRUE, if the path of the current page matches the site_frontpage configuration variable. This variables can be set from the "Site Information" configuration page.

Usually there is only a single front page on a Drupal site. If you use the Variable translation module (part of the Internationalization project), then you could translate the site_frontpage configuration variable and have a different front page for each enabled language.

It may also be possible to have other module alter the value of the variables on each request to have multiple front pages. Additional module may also alter the value of $is_front in all or only a few specific templates. However, I cannot think of any useful use case for this, except torturing the template developer with inconsistency.