[alabut]

How is it so intrusive to have 2 fields on a page?

Depends on the design, goals and intended audience.

Form fields are harder to ignore than most page elements and have a sort of gravitational pull on the eyeballs, so you can easily imagine situations where it's better to avoid them. Like, say, on public marketing pages for first time visitors to a microblogging app that's way hard to explain to your average non-blogging Yahoo-using grandma in flyover country.

[bmelton]

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.