Ed and Dispatches from the Culture Wars has some real information on the issue.
There are four really important facts here. First, that in the very few instances where actual fraud takes place (as opposed to duplicate voter registration applications or applications with portions that are illegible or have mistakes in them), ACORN is the victim of that fraud, not the perpetrator.
Second, that in almost every case where canvassers have turned in fraudulent applications, they were caught because ACORN turned in both the fraudulent card and the identity of the canvasser, and they do so separate from all the applications that they have no reason to believe are bad.
Third, that none of this has anything to do with anyone actually voting twice or anyone being registered to vote that is not a legal voter.
Fourth -- and this is very important -- the law requires ACORN to turn in every single registration card that they take, even if it's fraudulent. They cannot, by law, discard ANY voter registration application, they must turn them in to election officials. And bear in mind that they have little means of knowing if someone is turning in a duplicate application because they don't have access to the state's central voter file. If someone who is already registered fills out a card, either because they figure maybe their registration has lapsed and it won't hurt to make sure they're registered or because they feel sorry for a canvasser, by law those cards must be turned in to the county election officials.
So, when you hear the right whining and crying about this, it is based on a lie, a false issue that has nothing to do with actual votes case.