GAO has released this report.  From the executive summary:

Of 22 investigations and cases of election and voter registration fraud and wage violations involving ACORN or potentially related organizations from fiscal years 2005 through 2009, most were closed without prosecution. One of the eight cases and investigations identified by the Department of Justice resulted in guilty pleas by eight defendants to voter registration fraud and seven were closed without action due to insufficient, or a lack of, evidence. The Federal Election Commission (FEC) reported five closed matters; for one, the FEC reached a conciliation agreement with a penalty. Another matter was dismissed, and FEC found no reason to believe the violations occurred for three matters. The Department of Labor identified eight wage and hour disputes, plus one delinquent reporting of required documentation, all of which resulted in an organization complying or agreeing to take corrective measures to comply with the applicable requirements.