http://www.chuckherrin.com/hackthevote.htm
from Chuck Herrin, a professional security auditor, demonstrates how one could hack into the GEMS central vote tabulator system:
My paper shows with both commentary and images how easily an election could be lost as our fictional attacker breaks in and changes 11,000+ votes on an actual running installation of the Diebold Regional Vote Tabulation software. It includes before and after Official Election Results, screenshots, and timestamped audit logs that show no sign of any wrongdoing. I was in and out in about 5 minutes - it's so simple I hate to even call it "Hacking".