Maybe there is something I am missing here, but from the perspective of a theme guy, it seems that HTML comments shouldn't be considered content by the xen:contentcheck tag. I've noticed a few plugins will sometimes throw in HTML comments, and while this is nice, organized and dandy, it sometimes results in elements popping up that ideally shouldn't be hitting the front end (e.g. the user bar in UI.X). Just something to chew on. Please, offer your perspective and tell me if I am wrong.