Thanks massimo. The shield pictured above (v.1a) has a different layout than the one on Tom Igoe's site, and it says R1 and R2 on it while the resistors are called R1 and R7 on the shield on Tom's site. So I was going back and forth between all the different shield and Arduino board schematics, trying to make sense out of this.
Another thing which threw me off a little seems to be an error in the silk screening of the proto shield:
In the picture above the row of holes on the left says GROUND while they are actually connected to 5V, and the row on the right (which is supposedly +5V) is in fact connected to ground.
If you look carefully you can see the trace on the board going from the 5V pin on the bottom, underneath the printed word POWER to the vertical GROUND row on the left.
Me so confused!