I needed to create a drupal site where email addresses were usernames. Username settings in drupal don’t allow for email address characters, so setting the username to the email address wasn’t an option.
Like most things drupal, turns out there’s a module for that. It has a drupal 5, 6 and 7 version, and the 7 version works great on a site I’m working on. Drupal Email login: http://drupal.org/project/email_registration
It just adds some logic to the login function that looks up username based on email address and uses that with the core drupal login function.