It isn't only software or firmware that's of concern. There should be no compromise: everything should be transparent and therefor auditable. (You doubt the importance of this for voting machines ?) See http://it.slashdot.org/article.pl?sid=08/05/01/1233244
Voting machines are a very special case because you cannot trust the election authority to run them honestly. Using free software in these machines is not sufficient.
I do not trust computers for voting. Ballots should be on paper so that a recount is possible.