[Zope] user id naming conventions

Does anyone have any feelings on the best way to manage user id's? Specifically, I am trying to decide how to create user id's for CMF. I don't want users to have to create a user id themselves - seems like one more thing they have to keep track of and could deter people from using the site. Instead, I wanted to use email address as the id - it's unique and easy to remember. The problem, of course, is the '@' and '.' in email addresses. I could encode both chars at every point that user id is used - a hassle, but possible. The ZODB allows '@' in the id. Any thoughts?