Users are people objects, and by default people objects require first_names and last_name. You can disable this by removing the not null constraint for both columns on the persons table in the database (I work with sites that have done this and it works - although it makes dupe accounts much more likely). You will likely need to also update a number of other pages and procs, for example if posting on forums it trys to user you name to say who posted, or when on your my account page it asks for the first and last names... if you grep for "first_names" on your install you can see all the places that look for it...