I agree with Sebastian on this one. I also don't think the way you are going about this will result in a good thing. However, the module itself is already setup for a move to allow anonymous posting. First, the UserID of -1 will be properly handled by the system, it will display Anonymous on the front end anywhere UserID = -1 (this is to handle deleted users too). Second, the data store itself is already full prepared for this (meaning, no foreign key rules will be broken).
The only modifications you should need to make would involve the posts display (components\UI\Posts.vb, ModuleSearch.vb), the new thread buttons (Posts.vb, Threads.vb, ModuleSearch.vb) and Forum_PostEdit.ascx.vb. In all of the files, you need to remove any checks for Request.IsAuthenticated unless otherwise specified (or it doesn't make sense. ie. Showing edit post to an an unauthenticated user should never happen). In fact, throughout the code I even have comments about where and where not to do this for anonymous posting (although I am not sure it is 100% complete, it is in most areas). The only other codeI think you may need to review is within components\security. If you are separating start post permissions and reply permissions (basically, if your forum has posting restrictions) you will really need to review the ModuleSecurity.vb file in full (A forum without posting restirctions should never check for the IsAllowedToStartRestrictedThread or IsAllowedToPostRestrictedReply properties) .
The one thing I must note about this, there will be no on/off switch for this unless you change the permissions grid in the module (or add your own) so this will open the door for anonymous posting in all forums where an anonymous user can view the forum. Also, this is all off the top of my head so there may be something I am forgetting about.