Not mine but modified from here.
But if you pop this into functions.php or a plugin it will work.
add_filter( 'gettext', 'change_publish_button', 10, 2 );
function change_publish_button( $translation, $text ) {
if ( 'yourcustomposttype' == get_post_type())
if ( $text == 'Publish' )
return 'Save';
return $translation;
}