Traditionally we have allowed people to register without specifying an emailaddress. I also suspect that it is unlikely that we will be able to get rid of that tradition. It does however pose a problem in that if you don't specify an email address, there is no way to recover access to your account if you forget your password. This happens quite a bit. I see a comment on the helpdesk or the village pump every two weeks or so. And that's if people don't just 'give up'.
We used to have lots of textual warning language on the login page, which wasn't really too readable either and was removed in the login page redesign a couple of years ago.
But what if we, when the user registers, and leaves the email field blank, we give him an intermediate step. "Providing an e-mail address is optional, but you if you choose not to provide an email address, please be aware that there is no way to recover access to your account if you ever forget your password"
- I acknowledge that I cannot get access to my account if I forget my password
- input field for email