Since I *know* I’m not going to remember this:

When installing apps (like RT or Drupal) which expect a mysql 3.x server, on a mysql 4.x server, you get strange messages about being unable to log in, and suggested that you upgrade your mysql client.

Before you spend hours or days hunting for what this might mean, try this:

mysql> set password for username@localhost = OLD_PASSWORD(‘passwordhere’);
Query OK, 0 rows affected (0.01 sec)

That converts the password to the old format (whatever that means) and permits the login.