Hint where? I now see on http://en.wikipedia.beta.wmflabs.org/wiki/Special:PasswordReset :

Reset password
Fill in one of the fields to receive a temporary password via email.

Username:

Email address:

(In reply to Nemo from comment #1)

Hint where? I now see on
http://en.wikipedia.beta.wmflabs.org/wiki/Special:PasswordReset :

Reset password
Fill in one of the fields to receive a temporary password via email.

Username:

Email address:

The background: we (as sysops) have received some requests from users saying that they registered Wikipedia account with some Email address, but forgot the user name. To resolve this, users can (and this is what we can do anyway) use the password reset form and user names are mentioned in Emails for temporary passwords. However users are not told this fact anywhere.

Ok, updating summary. This is a regression caused by the (mostly) undocumented 1.22 changes to login: https://www.mediawiki.org/wiki/MediaWiki_1.22#Login_and_signup_forms

Up to 1.21, I see "Forgotten your login details?" next to the "Log in" button. Now the login button consumes all the horizontal space and has "Keep me logged in" above it, the newly introduced "Help with logging in" below it. I guess the only option is to add a "Forgot your username?" next to "Username" as done with t the password, in the cases where the space is not taken by the "Use secure connection" link.