There's a general article pointing to all sorts of writings about vote fraud, some more conspiratorial than others.
That said, I think that it's worthwhile to always strongly oppose any electronic voting system that does not have a verifiable paper trail.
On the other hand, this doesn't concern me so much on a personal level, because I live in Oregon, and we don't have electronic voting machines because we only do vote by mail! Woo-hoo!