There is no built-in mechanism, but a little creativity means you can do this just about anywhere in your code, from view.yml to layout.php to each individual action.
The view.yml method is easy enough:
apps/frontend/config/view.yml:
stylesheets: [main?v=<?php echo time() ?>, reset?v=<?php echo time() ?>, layout?v=<?php echo time() ?>]
Although I think this is a little too active, and I tend to use either the SVN revision or a overall project version number:
stylesheets: [main?v=<?php echo sfConfig('app_project_version') ?>, reset?v=<?php echo sfConfig('app_project_version') ?>, layout?v=<?php echo sfConfig('app_project_version') ?>]
where app_project_version
is set in apps/frontend/config/app.yml. Methods for layout.php and actionSuccess.php should be easy enough from here:
<?php use_stylesheet('blah?v='.sfConfig::get('app_project_version')); ?>