Yahoo have begun recycling old mail addresses. They have always had an expiry on non-Plus accounts (6 months I believe, account is deactivated, 12 months, account is deleted), but as far as I'm aware, the username would never get released. Now, they can be reclaimed by re-registering (by anyone). If you have any old Yahoo Mail accounts used as backup/password reset (e.g. on Amazon, iTunes, whatever), make sure you remove the Yahoo address as a backup address unless you intend on regularly logging into that Yahoo account to keep it alive, or you might have some nasty surprises when malicious password reset instructions go to the wrong person. I just tried it with an account that I know has been unused for just over a year, and was able to register it no problem.