Hi, Paul here... creator of the Teensy and the one responsible for adding the INPUT_PULLUP option in pinMode().
When I added this, I posted the idea and my code to the Arduino developer mail list. Several people discussed the idea, and ultimately it was rejected for inclusion into the official Arduino core. The Arduino team seemed to feel adding a third option unnecessarily complicated using the pinMode() function, and there was resistance to promoting "active low" logic because the concept of "0 means active, 1 means inactive" can be difficult for novices.
I have added "The INPUT_PULLUP option is a Teensy extension which is not present on the official Arduino" to the web page where this option is documented.
http://www.pjrc.com/teensy/td_digital.html
It's really a very simple extension that could very easily be added to the official Arduino core if they wanted it. I would be happy to prepare a patch.