You've heard the hype from McCain and others.
Here are the basic facts:
Yes, there are some questionable registration forms. There always are no matter who is doing the registration (any company).
What McCain and others are not telling you is that Acorn is required by law in many states to turn in ALL collected ballots, those that appear to be valid and those that don't. In some states they are allowed to help the state by flagging the ones in question. In other states, they are required to just turn them all in and the state tries to figure out which are valid and which are not.
So clearly much ado about nothing (other than yet another distraction rather than discussing the issues voters care about).