In brief, a law passed in 2005 meant to manage third party voter registration efforts is being challenged as overly restrictive and unconstitutional.
I am not sure about the constitutional arguments, but the defenders of the law are having a hard time defending some of its provisions, such as the requirement that voter registration forms be turned in in bundles of 50 and that all forms must be turned in within 48 hours of being collected. When asked where these requirements came from, one defender cited a single instance of 90 voter registration forms being stolen from an ACORN office in 2004. As to the bundling, no one seems to have a good reason for that requirement.
Violating the law does not only void the registration forms, but the individual or organization that collected the forms is subject to legal sanction.