My old computer security prof at UC Davis, Matt Bishop, was in the news. The University of California did a study on the security of California approved voting machines and it appears that the machines failed miserably. They were even hackable from the voting terminals themselves.
I'm sure that they are better than the machines used during the last presidential election, though. I did a project on voting machines for a computer ethics class about how bad the system was. Votes were tallied on node machines which were unprotected. The votes were tallied in an unencrypted text file. People in my class said they thought a big conspiracy of vote manipulation was far fetched. They missed the point that it would take just one person five minutes or less to change thousands of votes with notepad. Sorry.. too lazy to find links at the moment.