but you cant be 100% sure it just the button that needs replacing. The button connects to the motherboard, there's also chance that could be the thing that's buggered.
I fix ipods & iphones all day long, i've not come across this fault being caused by a faulty motherboard, however, I HAVE had these faults caused by liquid damage.
Yeah ive had it where both the water indicators (heaphone & charger socket) are clear, yet liquid has seeped into the side, its not allways a reliable indicator.