In the consumer world, there are some web sites which can take advantage of the anonymous nature of the web.
One of our clients, Linwood Manor, runs their web pages on their alcohol dependency treatment programme on the premise that the web creates a safe environment for people to speak up and ask for help in a way they might not be comfortable doing face to face or even over the phone. The fact that the web is anonymous and slightly impersonal actually helps their sign-up rate.
In Enterprise 2.0 the issue is somewhat different. Social software is all about showing off your profile and what you know. You want your contributions to discussions to be tracked back to you so that you are identified with the expertise that you demonstrate. Even so, there may be some areas where anonymity might be worthwhile, a safe place on the intranet to make off the wall suggestions, or even whistle-blowing where you want to encourage a similar environment to the external web.
Any organisation considering Enterprise 2.0 needs to think about how much anonymity they are prepared to allow, and how much will be beneficial or negative.