Voter fraud takes place at the polls. It doesn't need to be widespread to be serious.
Voter registration fraud takes place long before the vote. It occurs when fictional or ineligible people attempt to register. In order for this to be a problem of any significance, Mickey Mouse would not only have to make it on the voter rolls, but also make it to the polls to vote. That would be voter fraud. Short of that occurring, voter registration fraud is something akin to a prank phone call.
Fox News and the McCain campaign are trying to say that wide-spread voter fraud is occurring exactly three weeks to the day before the wide-spread opening of the polls.
BTW: This is being brought up to justify voter suppression which is, because it is a direct violation of the civil rights of the individual, as great, if not a greater betrayal of the ideals of democracy than voter fraud.