I noticed that your plugin would only work on the primary domain of my multisite and just figured out why:
on line 39 of fancybox.php you have:
define('FBFW_URL', WP_PLUGIN_URL . '/fancybox-for-wordpress');
however when using the domain mapping plugin this variable WP_PLUGIN_URL returns the original URL not the mapped one.
Reading on http://codex.wordpress.org/Determining_Plugin_and_Content_Directories I see where it mentions the constant WP_PLUGIN_URL there is a statement:
NOTE: These constants are *not SSL aware*. Apparently they are intended for Core use, not Plugins. Plugins should use the above functions instead, or use the compatibility code in the next section.
So this line should rather read:
define('FBFW_URL', plugins_url() . '/fancybox-for-wordpress');
Thanks for the plugin BTW