[The Arduino's ATmega has some built-in pullup resistors that can be enabled, to save you the trouble of adding them to your circuit. I don't recall for sure off the top of my head how to enable them - IIRC, you set it as an input, then write a 1 to it as if it were an output.
Wow! I had no idea there were internal pull-ups. I did a site search, and I couldn't find it documented anywhere. Anybody have a reference for that? If not, someone should add it to the documentation.