This sounds like one of those posts that is doomed to be the Dumbest Post Ever.... but I think the sample program at the bottom of the button shield tutorial is wrong. When I use that program as-is, the led is on when the button is not pressed, and turns off when I press the button. It looks like the problem is that the program is looking for pin 5 to be LOW when the button is pressed, while in reality the pin is normally LOW and pressing the button makes it go HIGH.
I checked & re-checked my soldering, checked the schematic, and tested the shield with my meter to make sure, and pin 5 really is normally LOW (pull-down resistor), and pushing the button makes it go HIGH.
I'd just change the wiki page, but I couldn't believe that no one has run across this yet so I'm checking here first. If it sounds like a legitimate change, I'll fix the wiki page. Thanks.