Why? It was a good post.
Because I realised afterwards that it wasn't exactly what you were suggesting, but a similar thing I had thought of.
For anyone else who was interested, my idea was to store a large, user-configurable string somewhere (by large, say 128 characters) in the config file, and use this appended to the password as part of the hashing process. In order to make passwords realistically crackable, an attacker would have to obtain data both from the database (the SQL data), and the file system (the key from the config file), which is a much more unlikely scenario than them just getting access via an SQL injection or something.
IPB could have a default value for this string, and power users could configure it (there would have to be some sort of column in the DB to dictate whether it had been applied if you were converting an existing system). It puts power users in a significantly better position in terms of password hashes and doesn't change the position of your regular users who don't bother to change it.