アップルから「Mac OS X 10.5: Unable to log in to account after an upgrade install」が出ていました。Leopardをインストールして、ユーザアカウントがおかしくなった場合の対処方法です。

ssue or symptom

You may not be able to log in with a user account that has a password of 8 or more characters and was originally created in Mac OS X 10.2.8 or earlier, after performing an upgrade installation of Mac OS X 10.5 Leopard (the default installation type).