TinyMCE in Livestreet 0.4.1 "tweaks" the inserted Youtube code
The solution turned out to be quite simple (an example is given for the default template new):
In files: We add the parameter media_strict: false to the initialization of TinyMCE. After that, the contents of the add.tpl files will look something like this:
/plugins/page/templates/skin/default/actions/ActionPage/add.tpl
/templates/skin/new/actions/ActionTalk/add.tpl
/templates/skin/new/actions/ActionTopic/add.tpltinyMCE.init({
...
media_strict : false,
...
});