The New York Times offers this must-read, front page report. A few snippets:

The remainder are registered voters who were changing their address and roughly 400,000 that were rejected by election officials for a variety of reasons, including duplicate registrations, incomplete forms and fraudulent submissions from low-paid field workers trying to please their supervisors, Mr. Slater acknowledged.
Most of the registrations that were rejected were duplicate forms, followed by incomplete forms. The Acorn officials said their investigation found about 9,000 voter registration cards that were determined to be fraudulent. A lawyer for the group estimated that perhaps 5,000 to 6,000 more cards employees turned in were fraudulent. Acorn officials said that 20 percent to 25 percent of the applications it submitted were likely duplicates, 5 percent were incomplete, and 1 percent to 1.5 percent were fraudulent. Mr. Slater said the estimates were based on past registration drives and a sampling of this one.