To the right of the password box, it says (click here for more information). Click there for more information!
This board has been relatively well-behaved, but the baby name board has erupted into chaos a few times over the last couple months. For the time being I will only require passwords from AOL users, who all share the same IPs, making it difficult to identify them. But regulars are encouraged to sign up now, to 'reserve' their usernames. Plus, as you'll notice in my message, the word 'guest' gets replaced with 'authenticated'. Tres chic!
And to answer Nanaea's question, the assigning of passwords is fully automated.