I've been following this drama for months, and it's an interesting illustration of how traditional First Amendment rights can be endangered when we place speech control in the hands of corporations. Scientology has waged a drawn-out war against Anonymous on YouTube, repeatedly filing DMCA takedowns which result in videos being pulled. Anonymous typically files counter-notices and the videos go back up. In the meantime, it seems likely that Scientology systematically breaks YouTube Terms of Service without enforcement on the part of YouTube. It is hard to tell for sure what the facts are, but this is my best understanding.
Over the last couple of days, unwarranted takedowns were executed by YouTube en masse for Anonymous videos, followed by reinstatement in most cases. EFF has a summary, and you can follow the Anonymous forums if you have the patience and stomach. It is to YouTube's credit that they appear to have restored videos/accounts in most cases, but it is also troubling that such a major deletion of speech could happen in the first place.