Is there a function to return all the multisite names?
Okay, this is what I am using for now in a drushrc.php file. If you can improve upon this please post a new answer. Thanks

// Gets a list of all directories in /sites.
$sites = glob(drush_locate_root() . '/sites/*', GLOB_ONLYDIR);
// Strips the absolute path except for the last name.
$sites = array_map('basename', $sites);
// Removes the 'all' folder.
$sites = array_diff($sites, array('all'));