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.
Sources:
http://edition.cnn.com/2008/POLITICS/10/09/acorn.fraud.claims/index.html
More detail but from a less objective source:
http://www.marketwatch.com/news/story/rnc-cnn-exposes-how-acorn/story.aspx?guid=%7BE0F68C91-381A-43E8-9857-D990EFC33B34%7D&dist=hppr
