I mean the Republicans quietly de-registering Democratic voters and making early votes for Obama disappear. This is from an article at afterdowningstreet.org:
"[McCain] and his campaign and allied supporters have sought to illegally remove hundreds of thousands of voters from the rolls, fraudulently registered people as Republicans without their knowledge and against their will, obstructed voter registration drives, falsely warned students against voting where they attend school, falsely accused community groups of voter registration fraud, falsely alleged the widespread existence of voter fraud, and encouraged supporters to falsely believe McCain's opponent is a foreign terrorist through speeches, recorded phone messages, and flyers. Already in early voting in a number of states there have been cases of votes on electronic machines visibly flipping to McCain or McKinney when intended for Obama. We will see McCain supporters on November 4th challenging people's right to vote, seeking to force people to vote on provisional ballots, and seeking to have provisional ballots discarded."
This is, I believe, corroborated by the investigations of people like Ted Kennedy. I would have no trouble accepting a fair vote, but it seems this one is already skewed.
As for Acorn, I don't find any hard evidence for them actually doing anything wrong, but feel free to show me.