Gov. Jim Doyle of Wisconsin signed into law a bill requiring any vendor who provides electronic voting machines must provide the code that runs on those machines.
This, combined with paper copies of the vote you cast for (which Wisconsin also requires) are a must when it comes to electronic voting. Not just to avoid the fraud as seen in Ohio in 2004, or as a check against corrupt CEOs who publicly say they’ll do everything in their power to see an election go a certain way and they also just happen to provide e-voting machines, but the code and paper voting provide trust to the end user that their vote counts.
Learn more about e-voting courtesy of the EFF.