I came across an interesting tidbit about overeager voter registration drives today. Evidently a community organizer umbrella group turned in 5000 voter registration forms in Gary, Indiana, of which the registrars found that the first 2100 were all fake before putting the rest aside to review potentially legitimate forms from other sources.
This is how the infamous Daley machine kept control of Chicago back in the 1960s, when death didn't terminate your right to vote.
One thing that occurred to me is that the electoral college tends to buffer Presidential elections from this kind of voting fraud. Since there are a lot of states with a bigger population than the popular vote edge in most presidential elections, massive voting fraud, even if local to just one of these states, could tip the popular vote. However, since the electoral vote tends to be a lot more lopsided, fraud in a limited geographical area is less likely to tip the electoral vote.
Made public on 20090509 since the election is over.
More detail but from a less objective source: