The fields on the login page share names with the fields on the registration page ('username', 'password'), resulting in some browsers autofilling those fields. Additionally, there appears to be a bug in some versions of Firefox (seems to be fixed in Firefox 13) that will aggressively send values for all fields (even those that may not exist on the form) if it detects some (say, 'password') fields that can be autofilled. This can become really annoying if the forum is run in a subfolder (example.com/phpBB) and a user already has an existing account on example.com with a saved username/password, if the really common name 'username' and 'password' are used for fields there as well.
The fix is to either rename the fields to something unique (for both the login page and registration page; say, 'phpbb_login_username' and phpbb_reg_username') or to just add autocomplete="off" to the username, password and email fields on the registration page. Renaming the fields is probably better as it allows for users to save different account credentials for a main site and forum, but this will break all styles. I would therefore suggest disabling autocomplete in 3.0.x and renaming the fields for 3.1.