It is my understanding that the www is actually a subdomain just like my in my.yahoo.com, but it is usually mapped to the main domain or vice versa. In other words www.domain.com and domain.com usually get you to the same place, but do not need to.
One problem I see with having both mapped to the same place is that a visitor might alternate between the two. If cookies are enabled for the site, then they will have two sets of cookies.