That makes sense, and says what I've felt/intuited (but never known for sure).

I typically put a username / password box on any pages I want people to be logged in for. Now, obviously, this includes pages that they must be logged in for, but those pages should enforce authentication more strictly; But what I really mean are the pages that don't 'require' authentication, but for any number of reasons, it is preferred.

Also, I've always believed that the 'two boxes on a page' approach draws them torwards the login, and acts more as a call to action than Twitter's dropdown approach.

As alabut says though, use where appropriate.