I've noticed that once I log on to YouTube, the dialog box stop appearing, and the videos just play in the webpage. Logging off again doesn't help, but once I clear the IE data it works again. If there is a cookie or some other data that gets stored I don't know, but it's, at least in my case, the logging on that breaks the playback through MC. I hope this can be fixed so MC can once again play YouTube videos when logged on.