They (and a number of others) have been making non-subvertable slot machines of various designs for decades! And they have this thing called the Nevada Gaming Board (now, since gam(bl)ing is basically universal, there are state Gaming Boards all over the place...but it all started there; they are the model). This august body is responsible for overseeing the manufacturers of gam(bl)ing devices, and I know from firsthand experience that they do not fuck around with anybody. You follow the rules, or you're out. Period. And one of the rules is that all your code is available for inspection at any time, and another is that you can only have in your machine code that has been fully certified as fair and balanced.

So any Diebold or anybody else who says it is too costly, or lofgistically impossible, or any of the other oh-so-lame excuses I've heard so far is just plain full of shit.