Quote from the FBI website - in particular, read last paragraph:
As of August 2006, the FBI has authorized use of the FBI Anti-Piracy seal and warning by members of the Motion Picture Association of America (MPAA), the Recording Industry Association of America (RIAA), the Software & Information Industry Association (SIIA), the Business Software Alliance (BSA) and the Entertainment Software Association (ESA), subject to each member entering into a formal Uniform Authorization Agreement. Association members should contact the MPAA, RIAA, SIIA, BSA, or ESA for more information.
Now we are evaluating the licensing arrangements we have with members of these associations with a view towards permitting the broadest possible public use of the seal by all individuals and businesses with a copyright interest. Please check this webpage periodically for updates on the status of this initiative.
In the meantime, it is perfectly fine for anyone, without FBI approval, to use the following generic language on material protected under U.S. copyright law: "Warning: The unauthorized reproduction or distribution of this copyrighted work is illegal. Criminal copyright infringement, including infringement without monetary gain, is investigated by the FBI and is punishable by up to 5 years in federal prison and a fine of $250,000."