It's always possible for a member to login with their email in place of their username. Still, sometimes you may not want to have a username field at all, and prefer to have people login just using their email address and password. Google, for example, does logins based on your email. To do this with WSN, go to Admin -> Settings -> Switches in your admin panel and turn on 'treat emails as usernames'.

Once on, your username won't work to login -- only your email.

Note that this switch works nicely in conjunction with the "autogenerate accounts" switch.