Salon has an article on the latest demonstration showing how voting machines -- in this case a Diebold model, though the basic approach is more broadly applicable -- can be easily and inexpensively modified to add switch a voter's choices after the voter has voted and pushed a button to confirm the votes cast.
This would require getting physical access to the machines probably before voting day, but as the article points out, this would not necessarily be difficult.
This is a far more practical and realistic form of vote fraud than having individuals vote under false identities, but Republicans in particular have been focusing on the latter despite the evidence that it's a non-problem. (Of course, people without up-to-date government-issued photo IDs tend to be elderly, poor, in bad health, or otherwise part of a group not known to lean Republican.)by