This article, published recently in Rolling Stone is worth reading. Kennedy describes unexpected results and occuring after a "patch" was secretly applied by Diebold staff in Georgia in 2002. In addition, he cites a study of the results of electronic voting systems in the 2004 election in Florida, showing possible evidence that the election results were altered. "The data show with 99.0 percent certainty that a county's use of electronic voting is associated with a disproportionate increase in votes for President Bush. Compared to counties with paper ballots, counties with electronic voting machines were significantly more likely to show increases in support for President Bush between 2000 and 2004." (See UC Berkeley Study Shows E-Voting 'Irregularities' in Florida)
Read Diebold's response to the article here and Mr. Kennedy's comments on their response here.
