In the consolidated suit, the Recording Industry Association of America accused Aimster, which has changed its name to Madster, of copyright infringement: Aimster/Madster, says the RIAA, allows the exchange of pirated material (check out Rolling Stone's coverage here).
Madster's defense -- unlike Napster, which was open to everyone, Madster limits its service to users of AOL's Instant Messenger (which, it should be noted, is itself open to everyone).
On Wedensday, Judge Marvin Aspen of the U.S. District Court for the Northern District of Illinois granted a preliminary injunction against Madster. Reuters reports that the judge instructed Madster to implement filtering software, "so that it does not infringe copyrighted works over its network."