A precedent setting ruling was passed by a New York judge, forcing Google (Blogger.com) to give up the name of a blogger who allegedly used defamatory remarks against an Australian model (link). This ruling means that all companies can be potentially at risk to have to to give up the identities of anyone who uses their web resources to make statements about other companies.
If for example you want to complain anonymously, about an airline that did you wrong, you had better have your facts straight or potentially, they can request your identity and take you to court for defamation. Whether or not you are right, the legal fees can be a deterrent against wronged individuals trying to take their story public.
It will be interesting to see where this leads.