Some Princeton researchers made a demonstration video of how it's possible to steal an election with a Diebold voting machine in under a minute. Anyone with physical access to the machine can put in malicious software to steal votes—such as election workers who have unsupervised access to the machines before elections. All they have to do is open up the machine with a key (or pick the lock), remove the old memory card, stick in your own memory card, boot the machine, and it automatically installs any software that was on the memory card.
At the end of the demonstration election, the poll machine prints out the incorrect "stolen election" result. The internal memory card also stores in the incorrect result. Every piece of evidence of how the election actually went reflects the "wrong" result. And, after the election is over, the vote stealing software can delete itself. There's no evidence left that the vote has been conducted incorrectly.
There's even a flaw in Diebold machines that allow a virus to spread from machine to machine, infecting a memory card and using it to spread to other machines.
Security Analysis of the Diebold AccuVote-TS Voting Machine [itpolicy.princeton.edu via Digg]