As to what it means, the ACLU says:

Once again, the Bush administration's unchecked authority has led to abuse and civil liberties violations," said ACLU Executive Director Anthony D. Romero. "The documents make clear that the Department of Defense may have secretly and illegally conducted surveillance beyond the powers it was granted by Congress. It also appears as if the FBI is serving as a lackey for the DoD in misusing the Patriot Act powers. At the very least, it certainly looks like the FBI and DoD are conspiring to evade limits placed on the Department of Defense's surveillance powers."

...."The expanded role of the military in domestic intelligence gathering is troubling. These documents reveal that the military is gaining access to records here in the U.S. – in secret and without any meaningful oversight," said Melissa Goodman, staff attorney with the ACLU's National Security Project. "There are real concerns about the use of this intrusive surveillance power."

If you're new to the issue of NSL's:

NSLs are secretly issued by the government to obtain access to personal customer records from Internet service providers, financial institutions, and credit reporting agencies. In almost all cases, recipients of the NSLs are forbidden, or "gagged," from disclosing that they have received the letters. While the FBI has broad NSL powers and compliance with FBI-issued NSLs is mandatory, the Defense Department's NSL power is more limited in scope, and, in most cases, compliance with Defense Department demands is not mandatory.

Background: The FBI is Spying on You and Me.