Lefty:
Some people wanting 'fool proof' button circuits will wire a series 200 ohm resistor from the I/O pin to the button switch as a extra safety measure, say from a human programming error where you set a pin to output by mistake (say setting it HIGH) and then the poor user presses the button wired to ground and poof goes the I/O pin. RuggedCircuits uses that in their ruggedized version of a arduino board.
My brain-fart was caused by a tutorial giving precisely that advice:
http://www.ladyada.net/learn/arduino/lesson5.html
Whats this 100? resistor all about? There's a 100? resistor we use to connect the input pin to either HIGH or LOW voltage. Why is it there? Well, lets say you accidentally set P2 to be an OUTPUT type pin, but then you connected it to 5V. If you write a LOW to the pin (0V) but its connected to HIGH (5V), you've basically caused a short circuit at that pin. This isn't very good for the pin and could damage it! The 100? resistor acts as a buffer, to protect the pin from short circuits.
(I'm not blaming the tutorial, but my faulty memory.)