I always liked using negative integers for objects that should only be created for special purposes. They stay out of the way of regular objects. Having an overloaded object_id "0" seems like a quick and easy way to run into problems. What is the reason that only one such account needs to exist? I really would not like to mix up anonymous for x application with anonymous for y application, because anonymous can have many different meanings.
Caroline makes a good point. However anonymous a user might need to be to certain audiences, they should never be untraceably anonymous, we all know we need to track users, even if they are transient, or need to make certain statements without everyone knowing who they are.
Additionally, an anonymous user may wish to have consistent identity on the website, even if it isn't associated with a valid email or real name. At some point in time they may wish to convert their account to a non-anonymous account.
If anonymous accounts exist, they should be easy to create, maybe requiring only a (screen)name and password. It might also be nice to add the ability of anonymous users to post without using cookies, by providing the name/password at the time of the post, or as a second step to posting without being logged in. However, this might make it a little too easy for a program to create many accounts just by posting to a single page.
