Yahoo! announced recently that it will be resetting all Yahoo! IDs that haven't been used for over 12 months come July 15. The move is an attempt to let new users get access to unused logins.
If you have an account that you want to keep around but haven't used in a while, all you have to do is log in to any Yahoo! product with the username you want to keep. Even if you don't need the account, it may be a good idea to log in anyway, to protect the rest of your online identity.
[email protected] Can Be Yours! [Yahoo!]