My real question is why would anyone ever use pull down resistors with a momentary switch?
It has been my experience that software-minded people tend to use pull-downs. They like the logic of "low = off, high = on." As Mike's write-up points out, pull-downs can be dangerous in some situations which is why hardware-minded generally use pull-ups.
I dunno. This software type hates having to put in extra resistors in his design. Of course, I solved the inversion problem by writing my own class library to cover different types of buttons, and did the inversion there, so in the normal code it will be non-zero for the button being pressed.