I came across this detailed article on problems with electronic voting systems by Jon Stokes: How to steal an election by hacking the vote (also available as PDF). Some of the academics working on detecting this problem are Walter Mebane and Jasjeet Sekhon. But of course, one can be subtler than this. The ACM (Association for Computing Machinery) is active in pointing out problems, and numerous articles can be found with a Google query.
Thanks for the not-very-encouraging posting; we need to know about such risks so we can work to avoid them.
For another, very real-world view with the same general conclusions, see Avi Rubin's blog on his day at the polls .