The Princeton University Center for Information Technology Policy has released a major research paper exposing the failings of Diebold voting machines. Here is the important portion of the executive summary:

The main findings of our study are: 1. Malicious software running on a single voting machine can steal votes with little if any risk of detection. The malicious software can modify all of the records, audit logs, and counters kept by the voting machine, so that even careful forensic examination of these records will find nothing amiss. We have constructed demonstration software that carries out this vote-stealing attack. 2. Anyone who has physical access to a voting machine, or to a memory card that will later be inserted into a machine, can install said malicious software using a simple method that takes as little as one minute. In practice, poll workers and others often have unsupervised access to the machines. 3. AccuVote-TS machines are susceptible to voting-machine viruses — computer viruses that can spread malicious software automatically and invisibly from machine to machine during normal pre- and post-election activity. We have constructed a demonstration virus that spreads in this way, installing our demonstration vote-stealing program on every machine it infects. 4. While some of these problems can be eliminated by improving Diebold's software, others cannot be remedied without replacing the machines' hardware. Changes to election procedures would also be required to ensure security.