Uh, why? I don't think anyone has seriously asserted that open source voting systems would be foolproof. On the other hand, we have actual cases where open source voting systems would have at least made serious flaws public before they were used for voting.
Besides, when you're talking "Trusting Trust", you have to worry just as much about the hardware you're voting on. Who designed and built that?
Don't let the perfect be the enemy of the good.